Use add/delete pattern instead of gets/cas.

Simpler & shorter code, fewer roundtrips, does not produce garbage on
python-memcached level (cas_ids).
Drawback: lock re-entry counter is not visible on memcached level.
Allow disabling release safety check.
Implement blocking acquisition quadratic backoff.
Add timeout support for lock acquisition.
Update class docstring.
Distinguish between unresponsive memcached and failed command.

python-memcached is making this hard for "add".
Implement stand-in optional behaviour.