hcm: also guard currentHostList with checkerMu
hcm: also guard currentHostList with checkerMu

Takes care of the following race:

	WARNING: DATA RACE
	Write at 0x0000018aeb20 by goroutine 188:
	  github.com/TykTechnologies/tyk.(*HostCheckerManager).UpdateTrackingList()
	      /home/mvdan/go/src/github.com/TykTechnologies/tyk/host_checker_manager.go:339 +0x30a
	  github.com/TykTechnologies/tyk.SetCheckerHostList()
	      /home/mvdan/go/src/github.com/TykTechnologies/tyk/host_checker_manager.go:544 +0x1225
	  github.com/TykTechnologies/tyk.TestHostChecker()
	      /home/mvdan/go/src/github.com/TykTechnologies/tyk/host_checker_test.go:98 +0x7e2
	  testing.tRunner()
	      /home/mvdan/tip/src/testing/testing.go:754 +0x16c

	Previous read at 0x0000018aeb20 by goroutine 17:
	  github.com/TykTechnologies/tyk.(*HostCheckerManager).StartPoller()
	      /home/mvdan/go/src/github.com/TykTechnologies/tyk/host_checker_manager.go:173 +0x34f
	  github.com/TykTechnologies/tyk.(*HostCheckerManager).CheckActivePollerLoop()
	      /home/mvdan/go/src/github.com/TykTechnologies/tyk/host_checker_manager.go:103 +0x212

Updates #822.

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