Fixing race conditions in zkconn.
Fixing race conditions in zkconn.

A client might be getting a conn from metaconn, and then
use it for queries. It might get closed at the same time.
So it needs to be protected by a mutex.

cloudbearings/vitess,tjyang/vitess,davygeek/vitess,xgwubin/vitess,dumbunny/vitess,davygeek/vitess,fengshao0907/vitess,kuipertan/vitess,HubSpot/vitess,kmiku7/vitess-annotated,rnavarro/vitess,atyenoria/vitess,mahak/vitess,mahak/vitess,nurblieh/vitess,fengshao0907/vitess,applift/vitess,dcadevil/vitess,SDHM/vitess,alainjobart/vitess,erzel/vitess,anusornc/vitess,yaoshengzhe/vitess,kmiku7/vitess-annotated,nurblieh/vitess,aaijazi/vitess,tjyang/vitess,tirsen/vitess,ngaut/vitess,anusornc/vitess,michael-berlin/vitess,yangzhongj/vitess,kmiku7/vitess-annotated,tjyang/vitess,skyportsystems/vitess,alainjobart/vitess,mattharden/vitess,pivanof/vitess,davygeek/vitess,yangzhongj/vitess,michael-berlin/vitess,tinyspeck/vitess,tinyspeck/vitess,vitessio/vitess,dumbunny/vitess,pivanof/vitess,mlc0202/vitess,tjyang/vitess,aaijazi/vitess,davygeek/vitess,HubSpot/vitess,tjyang/vitess,applift/vitess,cgvarela/vitess,tjyang/vitess,apmichaud/vitess-apm,rnavarro/vitess,cgvarela/vitess,dumbunny/vitess,alainjobart/vitess,atyenoria/vitess,cgvarela/vitess,mapbased/vitess,vitessio/vitess,tinyspeck/vitess,mahak/vitess,rnavarro/vitess,dumbunny/vitess,sougou/vitess,applift/vitess,aaijazi/vitess,tirsen/vitess,dcadevil/vitess,xgwubin/vitess,ptomasroos/vitess,aaijazi/vitess,AndyDiamondstein/vitess,pivanof/vitess,guokeno0/vitess,netroby/vitess,mlc0202/vitess,alainjobart/vitess,ptomasroos/vitess,cgvarela/vitess,netroby/vitess,fengshao0907/vitess,nurblieh/vitess,yaoshengzhe/vitess,HubSpot/vitess,mlc0202/vitess,pivanof/vitess,tinyspeck/vitess,atyenoria/vitess,HubSpot/vitess,kuipertan/vitess,tinyspeck/vitess,applift/vitess,pivanof/vitess,kmiku7/vitess-annotated,mattharden/vitess,kuipertan/vitess,rnavarro/vitess,alainjobart/vitess,applift/vitess,mlc0202/vitess,apmichaud/vitess-apm,alainjobart/vitess,mahak/vitess,applift/vitess,HubSpot/vitess,michael-berlin/vitess,mlc0202/vitess,SDHM/vitess,yaoshengzhe/vitess,anusornc/vitess,kmiku7/vitess-annotated,xgwubin/vitess,ptomasroos/vitess,aaijazi/vitess,skyportsystems/vitess,michael-berlin/vitess,ngaut/vitess,AndyDiamondstein/vitess,mlc0202/vitess,sougou/vitess,netroby/vitess,enisoc/vitess,erzel/vitess,SDHM/vitess,netroby/vitess,enisoc/vitess,apmichaud/vitess-apm,kuipertan/vitess,davygeek/vitess,ptomasroos/vitess,erzel/vitess,netroby/vitess,applift/vitess,ngaut/vitess,fengshao0907/vitess,SDHM/vitess,cgvarela/vitess,tirsen/vitess,erzel/vitess,nurblieh/vitess,alainjobart/vitess,kmiku7/vitess-annotated,vitessio/vitess,cgvarela/vitess,anusornc/vitess,enisoc/vitess,cgvarela/vitess,SDHM/vitess,atyenoria/vitess,mattharden/vitess,michael-berlin/vitess,fengshao0907/vitess,ngaut/vitess,aaijazi/vitess,cloudbearings/vitess,netroby/vitess,guokeno0/vitess,anusornc/vitess,dumbunny/vitess,ptomasroos/vitess,mlc0202/vitess,michael-berlin/vitess,AndyDiamondstein/vitess,kmiku7/vitess-annotated,pivanof/vitess,sougou/vitess,atyenoria/vitess,dumbunny/vitess,skyportsystems/vitess,skyportsystems/vitess,tinyspeck/vitess,yangzhongj/vitess,dumbunny/vitess,skyportsystems/vitess,pivanof/vitess,vitessio/vitess,applift/vitess,AndyDiamondstein/vitess,kuipertan/vitess,yangzhongj/vitess,SDHM/vitess,yaoshengzhe/vitess,enisoc/vitess,dumbunny/vitess,vitessio/vitess,dumbunny/vitess,alainjobart/vitess,mahak/vitess,netroby/vitess,skyportsystems/vitess,kuipertan/vitess,vitessio/vitess,ngaut/vitess,ptomasroos/vitess,guokeno0/vitess,xgwubin/vitess,xgwubin/vitess,yaoshengzhe/vitess,dcadevil/vitess,nurblieh/vitess,erzel/vitess,sougou/vitess,tirsen/vitess,vitessio/vitess,mahak/vitess,erzel/vitess,guokeno0/vitess,sougou/vitess,AndyDiamondstein/vitess,HubSpot/vitess,dcadevil/vitess,rnavarro/vitess,yangzhongj/vitess,cloudbearings/vitess,tjyang/vitess,rnavarro/vitess,mattharden/vitess,mapbased/vitess,guokeno0/vitess,rnavarro/vitess,skyportsystems/vitess,cloudbearings/vitess,enisoc/vitess,mapbased/vitess,mapbased/vitess,yaoshengzhe/vitess,mahak/vitess,aaijazi/vitess,guokeno0/vitess,pivanof/vitess,apmichaud/vitess-apm,yangzhongj/vitess,yangzhongj/vitess,ptomasroos/vitess,mattharden/vitess,pivanof/vitess,cloudbearings/vitess,kmiku7/vitess-annotated,anusornc/vitess,anusornc/vitess,mapbased/vitess,mattharden/vitess,atyenoria/vitess,yangzhongj/vitess,xgwubin/vitess,anusornc/vitess,dcadevil/vitess,netroby/vitess,sougou/vitess,mattharden/vitess,xgwubin/vitess,xgwubin/vitess,cloudbearings/vitess,SDHM/vitess,davygeek/vitess,skyportsystems/vitess,AndyDiamondstein/vitess,nurblieh/vitess,michael-berlin/vitess,dumbunny/vitess,nurblieh/vitess,enisoc/vitess,tirsen/vitess,mattharden/vitess,tirsen/vitess,apmichaud/vitess-apm,applift/vitess,aaijazi/vitess,HubSpot/vitess,skyportsystems/vitess,AndyDiamondstein/vitess,mapbased/vitess,mapbased/vitess,aaijazi/vitess,fengshao0907/vitess,cgvarela/vitess,kuipertan/vitess,guokeno0/vitess,guokeno0/vitess,mlc0202/vitess,tirsen/vitess,michael-berlin/vitess,enisoc/vitess,rnavarro/vitess,kuipertan/vitess,tjyang/vitess,mahak/vitess,mattharden/vitess,atyenoria/vitess,erzel/vitess,tjyang/vitess,ptomasroos/vitess,yaoshengzhe/vitess,vitessio/vitess,yao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