db/repl_block.h opReplicatedEnough has to be called in _recvChunkCommit

Here's a race:

> 0) TO-Primary receives last mods for a transfered chunk
> 1) FROM-Primary initiates chunk migration commit calling
> _recvChunkCommit on TO-Primary
> 2) TO-Primary acks it but TO-Secondary hasn't caught up
> 3) FROM-Primary updates config
> 4) TO-Primary dies
> 5) TO-Secondary takes over but it doesn't know about last mods