Improve Coordinator.Stop / Task Manager Shutdown

[schmichael]

As of 8f870a5 taskmgr should never have to release a task during stop().

This code should probably log or panic or something:

metafora/m_etcd/taskmgr.go

Lines 179 to 180 in 8f870a5

	default:
	close(states.release)

Or do nothing except block on the waitgroup?

Alternate to fix embedded coordinator

Update: Fixed by #98

There's a race with the embedded coordinator where the consumer shutting down causes a deadlock because embedded's Release blocks on the released task being claimed again or Close being called.

Close won't be called until the task is Released though! So the embedded coordinator either needs fixing or we need to reevaluate the relationship between Coordinator.Close() and task releasing.