Revert "Refactored creation of nats handler to return a pointer to a connected nats handler"

The original refactor caused the agent to exit early since it no longer
blocked when running the handler.

This reverts commit 789e259a236791dce4d81ab34357bbfa254c065b.

Signed-off-by: Nick Wade <nwade@pivotallabs.com>

Conflicts:
	go_agent/src/bosh/agent/agent.go
	go_agent/src/bosh/agent/agent_test.go
	go_agent/src/bosh/mbus/fakes/fake_handler.go
	go_agent/src/bosh/mbus/handler_interface.go
	go_agent/src/bosh/mbus/nats_handler.go
	go_agent/src/bosh/mbus/nats_handler_test.go
move TaskValue inline and unexported into dispatcher