Looks like clients/hash behaves incorrectly when dead server comes back alive - it's never removed from dead_clients and thus retried indefinitely.

Failing test and fix are visible in 35ee9de, rest of PR simply adds some test coverage and refactors existing code a little.

Prepare the version 3.0.1 release

Our current MockMemcacheClient doesn't throw an error if a key has a
space, and given MockMemcacheClient can be consumed by downstream
things, we want it to act as close to the real client as possible.

Rename _check_key to check_key_helper so we aren't importing a private
function.

pass the encoding from the hashed client to the pooled client when use_pooling is True.