Fix data race in the host checker
Fix data race in the host checker

StartPoller() will start its own goroutine and StopPoller doesn't loop,
so both goroutines here are redundant. In fact they introduce a race:

	Write at 0x00c4202564e9 by goroutine 70:
	  github.com/TykTechnologies/tyk.(*HostUptimeChecker).ResetList()
	      /home/mvdan/go/src/github.com/TykTechnologies/tyk/host_checker.go:257 +0x40
	  github.com/TykTechnologies/tyk.(*HostCheckerManager).UpdateTrackingList()
	      /home/mvdan/go/src/github.com/TykTechnologies/tyk/host_checker_manager.go:346 +0x674
	  [...]

	Previous write at 0x00c4202564e8 by goroutine 47:
	  github.com/TykTechnologies/tyk.(*HostCheckerManager).StartPoller()
	      /home/mvdan/go/src/github.com/TykTechnologies/tyk/host_checker_manager.go:169 +0x9cb

This is because we might use the poller before it's finished starting
up.

Updates #363.

lonelycode/tyk,nebolsin/tyk,lonelycode/tyk,nebolsin/tyk,mvdan/tyk,nebolsin/tyk,mvdan/tyk,mvdan/tyk,nebolsin/tyk,nebolsin/tyk,lonelycode/tyk,mvdan/tyk,mvdan/tyk,nebolsin/tyk,nebolsin/tyk,mvdan/tyk,nebolsin/tyk,mvdan/tyk,mvdan/tyk