Add inspection commands: keys, count, volume, stats - #4

Summary

Added four read-only inspection commands related to cache: keys, count, volume, stats

Commands

• keys - list all keys in the cache including the expired keys as well
• count - number of items in the cache, uses __len__ under the hood.
• volume - gives the total on-disk size of the cache, displays in KBs/MBs
• stats - hits/misses tuple from Cache.stats(), deliberately kept read-only here as --reset is not exposed.

Notes

• cmd_keys(cache, args) does not need to handle the Timeout exception, so no handle_timeout decorator used for that.
• the method signature remains same for these four commands as others, to be compatible with the handler(cache, args) method in main()
• modified the handle_timeout() decorator to be compatible with the args where 'key' does not exists.

Testing

• Added 6 new tests to validate the implementation for these commands, including the check for the modified decorator to handle the args without keys
• All tests passing with the coverage of 98.32%, above the project spec of 98%.