* [Feature] Added a map collection type.
* [Feature] Showed the current prefix count with stats command.
* [Feature] Combined the commands for getting/setting configuration
into a single config command.
* [Feature] Set the maximum sticky memory with sticky_limit config.
* [Feature] Printed the invalid command if bad commad error occurs.
* [Feature] Stopped cache node if more than one server mapping znodes exist.
* [Fix] Fixed key dump bugs such as memory leak, duplicate dump, etc.
* [Fix] Fixed the negative free_chunk_space problem.
* [Enhancement] Reduced excessive evictions by adjusting sm classes.
* [Enhancement] Enhanced set performance by using prefix pointer
when check invalid items.
* [Internal] Managed engine source code with separate engine directories.
* [Internal] Added the demo engine for use as a reference engine.
* [Internal] Performed the preparation task for long key support.
* [Internal] Performed ZK operation outside of zk watcher with a separate thread.
* [Internal] Added/used a scan facility to scan items in hash table.
* [Internal] Modified the ketama hash ring structure more usefully.
* [Other] Extensive minor enhancements.
* [Other] Extensive code refactoring.
