Currently if limit=0 is passed to lock_path then the method will time
out and never acquire a lock which is reasonable but not useful. This
is a potential pitfall given that in other contexts, for example
diskfile's replication_lock, a concurrency value of 0 has the meaning
'no limit'. It would be easy to erroneously assume that the same
semantic holds for lock_path.
To avoid that pitfall, this patch makes it an error to pass limit<1 to
lock_path.
Related-Change: I3c3193344c7a57a8a4fc7932d1b10e702efd3572
Change-Id: I9ea7ee5b93e3d6924bff9790141b107b53f77883