ctdbd: locking: Provide non-blocking API for locking of TDB record/db/alldb

ctdbd: locking: Provide non-blocking API for locking of TDB record/db/alldb

This introduces a consistent API for handling locks on single record, complete
db or all dbs. The locks are taken out in a child process. In cases of timeout,
find the processes that currently hold the lock and log.

Callback functions for locking requests take locked boolean to indicate
whether the lock was successfully obtained or not.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>