refactor(registry): Use mutexes rather than TTLd locks

Replace old 'claim' heuristic with a 'lock', yielding
a mutex. All callers that attempt to lock a resource
will now unlock their mutex proactively rather than
depending on the TTL.

Fix #102
Merge pull request #124 from bcwaldon/fix-102

refactor(registry): Use mutexes rather than TTLd locks