Per device replication lock

New replication_one_per_device (True by default)
that restricts incoming REPLICATION requests to
one per device, replication_currency allowing.

Also has replication_lock_timeout (15 by default)
to control how long a request will wait to obtain
a replication device lock before giving up.

This should be very useful in that you can be
assured any concurrent REPLICATION requests are
each writing to distinct devices. If you have 100
devices on a server, you can set
replication_concurrency to 100 and be confident
that, even if 100 replication requests were
executing concurrently, they'd each be writing to
separate devices. Before, all 100 could end up
writing to the same device, bringing it to a
horrible crawl.

NOTE: This is only for ssync replication. The
current default rsync replication still has the
potentially horrible behavior.