doc: add etcd-lock into libraries-and-tools.md

Lock implementation for etcd. It has three go routines:
    a) acquire - loop that watches for the lock to be free and tries to acquire it.
    b) watch - to watch for lock changes
    c) refresh - to refresh the ttl when the lock is acquired

All the changes in lock ownership are notified on the events channel. Any feedback welcome!
Merge pull request #1815 from ravigadde/patch-1

docs: add etcd-lock into libraries-and-tools.md