Replace SemispaceCache with LRU Cache

[mpilquist]

This PR removes SemispaceCache and adds a least recently used cache (creatively named Cache). This improves cache behavior, especially with access patterns where cached entries were promoted from gen 1 to gen 0 and resultantly lots of cached statements would be evicted.

The LRU implementation is based on maps and access stamps -- not particularly storage efficient but should be good enough for this use case. I thought about implementing this with a splay tree instead but opted for the hash map design.

The configured max cache space is accurately enforced now, whereas SemispaceCache would store up to twice the configured max. Hence, the session defaults were all doubled to get the same net maximum.

Eviction tracking is NOT built in to Cache like it was in SemispaceCache. Instead, putting an element in to a cache returns the element that was evicted as a result, if any. Eviction tracking moved up in to StatementCache instead.

cc @matthughes

[matthughes]

LGTM. I built and ran this PR against my integration tests and regular runtime and did not see any issues. Confirmed that it does not create more pg_prepared_statements than cache limit.