Explicit defragmentation of ep_engine buckets

As discussed in the parent (MB-10496) task, and in https://docs.google.com/document/d/1sgE9LFfT5ZD4FbSZqCuUtzOoLu5BFi1Kf63R9kYuAyY/edit# it has been demonstrated that there is significant memory usage overhead (over mem_used) due to the chunk-based memory allocation used by malloc - see attached graph.

We should investigate reducing this overhead by performing explicit defragmentation of ep_engine's in-memory hash table.