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.

Change-Id: I36e99a3d7e100699c76db6d3a4846514537ff685

etc/object-server.conf-sample

@@ -105,6 +105,16 @@ use = egg:swift#object
 # Note that REPLICATION is currently an ssync only item
 # replication_concurrency = 4
 #
+# Restricts incoming REPLICATION requests to one per device,
+# replication_currency above allowing. This can help control I/O to each
+# device, but you may wish to set this to False to allow multiple REPLICATION
+# requests (up to the above replication_concurrency setting) per device.
+# replication_one_per_device = True
+#
+# Number of seconds to wait for an existing replication device lock before
+# giving up.
+# replication_lock_timeout = 15
+#
 # These next two settings control when the REPLICATION subrequest handler will
 # abort an incoming REPLICATION attempt. An abort will occur if there are at
 # least threshold number of failures and the value of failures / successes