Adds scoring and single-worst-candidate eviction - #15

Summary

Adds scoring for sampled candidates, and evicition of a single worst candidate from those sampled candidates.

Changes

• _scoring(candidates) scores the selected sample of candidates based on accessed_count/max(size, 1), and returns the candidate with worst(low) score. The 0 size case is handled, and returns None for an empty candidates list.
• _evict_worst(candidates) removes the worst-scoring candidate now. Also cleans up the file associated with it, if any. Follow the same patters as existing __delitem__ method.
• Deliberately did not use the retry=True in the transaction in the _evict_worst as retry loop has no cap on blocking time, if the lock is held by another writer. It will fight the planned graceful shutdown for the background thread (upcoming PRs).

Testing

• Adds 6 tests, and runs successfully.
• Complete test-suite passes successfully without any modification, test coverage is above 98%