Add remove, computeIfAbsent and keys() (#1)

Squash merge of #1.

Add CHANGELOG entry for v1.2.0 cache statistics feature

Add edge-case tests for invalid capacity, expired-but-unpurged size c…

…ounting, and key re-put

Adds 3 new regression tests: constructor rejection of capacity < 1,
size() counting an expired-but-not-yet-purged entry per the documented
contract, and re-putting an existing key not evicting other entries.
All existing and new tests pass (including concurrency tests); no bugs
were found.

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