discovery: Exit early from a sleep when the context is done/canceled.
discovery: Exit early from a sleep when the context is done/canceled.

Since Close() is waiting for all Go routines to finish now, unit tests block much longer now when they retry on an incomplete tablet (e.g. which doesn't run the fake query service). Instead of waiting for the next retry (usually 5 seconds), we'll give up sooner now.
mattharden/vitess,applift/vitess,vitessio/vitess,mapbased/vitess,mapbased/vitess,mahak/vitess,HubSpot/vitess,pivanof/vitess,mattharden/vitess,mahak/vitess,enisoc/vitess,applift/vitess,mattharden/vitess,mahak/vitess,tinyspeck/vitess,davygeek/vitess,tirsen/vitess,dcadevil/vitess,mattharden/vitess,dcadevil/vitess,vitessio/vitess,sougou/vitess,tinyspeck/vitess,alainjobart/vitess,mahak/vitess,vitessio/vitess,pivanof/vitess,sougou/vitess,mattharden/vitess,HubSpot/vitess,sougou/vitess,mapbased/vitess,erzel/vitess,dcadevil/vitess,tirsen/vitess,applift/vitess,tinyspeck/vitess,pivanof/vitess,sougou/vitess,davygeek/vitess,erzel/vitess,vitessio/vitess,rnavarro/vitess,HubSpot/vitess,enisoc/vitess,enisoc/vitess,pivanof/vitess,sougou/vitess,mapbased/vitess,pivanof/vitess,applift/vitess,davygeek/vitess,alainjobart/vitess,HubSpot/vitess,sougou/vitess,alainjobart/vitess,davygeek/vitess,dumbunny/vitess,mapbased/vitess,alainjobart/vitess,dcadevil/vitess,dumbunny/vitess,pivanof/vitess,dumbunny/vitess,tinyspeck/vitess,erzel/vitess,mapbased/vitess,rnavarro/vitess,applift/vitess,alainjobart/vitess,rnavarro/vitess,davygeek/vitess,enisoc/vitess,rnavarro/vitess,rnavarro/vitess,applift/vitess,tinyspeck/vitess,erzel/vitess,dcadevil/vitess,tinyspeck/vitess,mapbased/vitess,pivanof/vitess,sougou/vitess,mattharden/vitess,dumbunny/vitess,mapbased/vitess,HubSpot/vitess,tirsen/vitess,tirsen/vitess,tirsen/vitess,mahak/vitess,erzel/vitess,rnavarro/vitess,HubSpot/vitess,vitessio/vitess,applift/vitess,rnavarro/vitess,davygeek/vitess,vitessio/vitess,vitessio/vitess,alainjobart/vitess,vitessio/vitess,tirsen/vitess,applift/vitess,mapbased/vitess,applift/vitess,dcadevil/vitess,enisoc/vitess,tirsen/vitess,pivanof/vitess,dumbunny/vitess,erzel/vitess,mattharden/vitess,sougou/vitess,pivanof/vitess,rnavarro/vitess,dumbunny/vitess,applift/vitess,dumbunny/vitess,erzel/vitess,dumbunny/vitess,alainjobart/vitess,HubSpot/vitess,mahak/vitess,mattharden/vitess,mahak/vitess,dumbunny/vitess,mahak/vitess,mattharden/vitess,enisoc/vitess,tinyspeck/vitess,dcadevil/vitess,erzel/vitess,HubSpot/vitess,dumbunny/vitess,tirsen/vitess,erzel/vitess,erzel/vitess,davygeek/vitess,alainjobart/vitess,tirsen/vitess,mapbased/vitess,mattharden/vitess,enisoc/vitess,rnavarro/vitess