Fix LeaderAppointer check for elastic.

Taking the coward way out of this. Loosing the partition count check on
LeaderAppointer promotion. If a new LeaderAppointer is elected, only crash if
the number of partitions is LESS than that when the cluster initially
started. Elastic join adds new partitions on the fly, so tolerate that.
ENG-4622: Allow setting timeout on the SimpleClientResponseAdapter's sync
callback.