Memcached backend: batched `delete_many`, fewer thread hops, exact `get_and_delete`

[allen0099]

Memcached backend improvements. Not breaking.

The issue first said clear() swallows exceptions; on current master it is clear_path() that does (#177).

Split into one sub-issue per pull request:

• Memcached: override delete_many with a batched delete #174
• Memcached get_and_delete can return a value it did not delete #175
• Memcached: run each multi-step operation in one worker call #176
• Memcached clear_path hides connection failures #177

[allen0099]

All sub-issues are done; closing.