jqueue: make replication on job update optional

Sometimes it's useful to write to the local filesystem, but immediate
replication to all master candidates is not needed.

The _WriteAndReplicateFileUnlocked function gets renamed to
_UpdateJobQueueFile, as calling "write and replicate, but don't
replicate" seemed a bit strange.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>