Move getMeta in to memcached

getMeta is implemented in ep-engine under process_unknown_command. We should be able to get enough information from bucket_get_item_info to do this in memcached for both deafult and ep-engine.

Context

getMeta is a command which was originally specific to ep_engine - no other engine implemented it. As such, memcached front-end didn't know about this command. It was implemented by using the unknown_command interface in engine.h - the raw packet was passed to ep_engine, which was decoded, operation executed, and then response encoded by ep_engine itself and a simple status code returned to memcached.

As part of MB-25422, it was necessary for us to implement getMeta additionally for default engine (aka memcache bucket type). This was done by essentially duplicating the code from ep-engine into default engine - see the linked commit for MB-25422. At the time we did this because we needed a quick way to fetch metadata for tests, to verify a Spock bug.

Task

We don't want to have code duplicated between ep_engine and default_engine; and moreover where possible we want to have memcached be responsible for packet decoding, verification, so the engines themselves don't need to concern themselves with request / response encoding. We therefore want to move the common aspects of these functions - the packet decode, verification, and response encoding - into memcached, and have the engines just implement the actual fetching of the metadata.