Reinstate the multiraft writeTask.

This gets us back to the state we were in before the move to etcd.

@spencerkimball @cockroachdb/developers 
It's the actual command; this is the "apply command to state machine" step. etcd/raft currently returns an array of CommittedEntries because it holds all the entries in memory; when raft no longer keeps everything in memory it could just return the high watermark log index. I'll add a note to cockroachdb/etcd#2