Make PooledClient.gats and gets fallbacks match Client - #666

Follow up to #642, which fixed PooledClient.gets but left PooledClient.gats behind.

• PooledClient.gats now takes cas_default and passes expire, default and cas_default through to Client.gats, matching the Client.gats signature and return type.
• With ignore_exc=True, PooledClient.gats returned the bare default on error instead of a (value, cas) tuple, which broke value, cas = client.gats(key). It now returns (default, cas_default).
• With ignore_exc=True, PooledClient.gets returned (None, None) on error and ignored the caller's defaults. It now returns (default, cas_default).

Tests: added TestPooledClient tests for gats (found, not found with defaults) and for the ignore_exc error path of gets and gats. The error path tests use an inner client that raises, since the pooled clients are always created with ignore_exc=False. The full unit and integration suite passes.

Fixes #665