Add maintenance commands: clear, expire, cull, evict, check - #5

Summary

Adds five maintenance and consistency related commands: clear, expire, cull, evict, check

Commands added

• clear - remove all the items from the cache, and prints the count of items removed
• expire - remove already-expired items only, and prints their count
• cull - remove items as per the set eviction policy until the volume comes under size_limit, also prints the count of removed items
• evict TAG - remove all items corresponding to a particular tag, and prints their count
• check [--fix] - verify DB/filesystem consistency, prints warning messages if any exists, or confirms consistency. Also runs 'VACUUM' to fix the inconsistency if the --fix flag is turned on.

Notes

• used explicit timeout handling for the cmd_evict as it returns the count even when exception occurs.
• No confirmation prompts added for the destructive commands (will be added in subsequent PRs)

Testing

• Added 8 new tests, all passing with the existing ones.
• Coverage is at 98.35%, above project's spec of 98%