This commit replaces boolean replication_one_per_device by an integer
replication_concurrency_per_device. The new configuration parameter is
passed to utils.lock_path() which now accept as an argument a limit for
the number of locks that can be acquired for a specific path.
Instead of trying to lock path/.lock, utils.lock_path() now tries to lock
files path/.lock-X, where X is in the range (0, N), N being the limit for
the number of locks allowed for the path. The default value of limit is
set to 1.
Change-Id: I3c3193344c7a57a8a4fc7932d1b10e702efd3572
e199192cae