Primarily a cleanup commit, however, currently there is a potential race
condition (that I'm not sure we've ever actually hit) during a flapping
SetEnabled/Disabled state where we may never correctly restart the eval
broker, if it was being called from multiple routines.

Currently when an evalbroker is disabled, it still recieves delayed
enqueues via log application in the fsm. This causes an ever growing
heap of evaluations that will never be drained, and can cause memory
issues in larger clusters, or when left running for an extended period
of time without a leader election.

This commit prevents the enqueuing of evaluations while we are
disabled, and relies on the leader restoreEvals routine to handle
reconciling state during a leadership transition.

Existing dequeues during an Enabled->Disabled broker state transition are
handled by the enqueueLocked function dropping evals.