Fix data race in UpdateStreamControlMock.
Fix data race in UpdateStreamControlMock.

Showed up in wrangler/testlib with go test -race.

tjyang/vitess,erzel/vitess,mapbased/vitess,dumbunny/vitess,vitessio/vitess,rnavarro/vitess,pivanof/vitess,erzel/vitess,guokeno0/vitess,AndyDiamondstein/vitess,enisoc/vitess,mapbased/vitess,pivanof/vitess,erzel/vitess,HubSpot/vitess,tjyang/vitess,erzel/vitess,mapbased/vitess,mapbased/vitess,applift/vitess,dcadevil/vitess,pivanof/vitess,dumbunny/vitess,tinyspeck/vitess,mattharden/vitess,dcadevil/vitess,sougou/vitess,tjyang/vitess,aaijazi/vitess,guokeno0/vitess,yaoshengzhe/vitess,mapbased/vitess,AndyDiamondstein/vitess,rnavarro/vitess,applift/vitess,tirsen/vitess,AndyDiamondstein/vitess,dumbunny/vitess,alainjobart/vitess,davygeek/vitess,rnavarro/vitess,dumbunny/vitess,alainjobart/vitess,dumbunny/vitess,AndyDiamondstein/vitess,tjyang/vitess,rnavarro/vitess,tinyspeck/vitess,yaoshengzhe/vitess,rnavarro/vitess,vitessio/vitess,mahak/vitess,enisoc/vitess,AndyDiamondstein/vitess,HubSpot/vitess,mahak/vitess,mattharden/vitess,mapbased/vitess,mattharden/vitess,enisoc/vitess,davygeek/vitess,erzel/vitess,erzel/vitess,dumbunny/vitess,alainjobart/vitess,alainjobart/vitess,applift/vitess,mattharden/vitess,erzel/vitess,tirsen/vitess,tjyang/vitess,applift/vitess,davygeek/vitess,mapbased/vitess,yaoshengzhe/vitess,guokeno0/vitess,pivanof/vitess,aaijazi/vitess,dcadevil/vitess,rnavarro/vitess,mahak/vitess,vitessio/vitess,HubSpot/vitess,guokeno0/vitess,applift/vitess,tirsen/vitess,tirsen/vitess,aaijazi/vitess,mapbased/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,sougou/vitess,tjyang/vitess,mattharden/vitess,tjyang/vitess,alainjobart/vitess,rnavarro/vitess,yaoshengzhe/vitess,AndyDiamondstein/vitess,enisoc/vitess,mattharden/vitess,mattharden/vitess,pivanof/vitess,pivanof/vitess,HubSpot/vitess,guokeno0/vitess,tinyspeck/vitess,AndyDiamondstein/vitess,AndyDiamondstein/vitess,dcadevil/vitess,sougou/vitess,applift/vitess,dcadevil/vitess,enisoc/vitess,mapbased/vitess,guokeno0/vitess,guokeno0/vitess,AndyDiamondstein/vitess,dumbunny/vitess,aaijazi/vitess,davygeek/vitess,enisoc/vitess,vitessio/vitess,erzel/vitess,tirsen/vitess,tirsen/vitess,guokeno0/vitess,aaijazi/vitess,tirsen/vitess,tirsen/vitess,dumbunny/vitess,applift/vitess,tinyspeck/vitess,mahak/vitess,sougou/vitess,tinyspeck/vitess,HubSpot/vitess,pivanof/vitess,tirsen/vitess,mattharden/vitess,erzel/vitess,mattharden/vitess,erzel/vitess,mahak/vitess,rnavarro/vitess,aaijazi/vitess,yaoshengzhe/vitess,tinyspeck/vitess,aaijazi/vitess,tinyspeck/vitess,dumbunny/vitess,dcadevil/vitess,alainjobart/vitess,vitessio/vitess,sougou/vitess,aaijazi/vitess,applift/vitess,applift/vitess,sougou/vitess,HubSpot/vitess,applift/vitess,davygeek/vitess,dumbunny/vitess,yaoshengzhe/vitess,HubSpot/vitess,tjyang/vitess,enisoc/vitess,vitessio/vitess,sougou/vitess,HubSpot/vitess,yaoshengzhe/vitess,yaoshengzhe/vitess,sougou/vitess,aaijazi/vitess,pivanof/vitess,vitessio/vitess,davygeek/vitess,mattharden/vitess,mahak/vitess,tjyang/vitess,pivanof/vitess,alainjobart/vitess,davygeek/vitess,dcadevil/vitess,yaoshengzhe/vitess,rnavarro/vitess,mapbased/vitess,guokeno0/vitess,alainjobart/vitess