vinyl: add global memory stats

box.info.memory() gives you some insight on what memory is used for,
but it's very coarse. For vinyl we need finer grained global memory
statistics.

This patch adds such: they are reported under box.stat.vinyl().memory
and consist of the following entries:

 - level0: sum size of level-0 of all LSM trees.
 - tx: size of memory used by tx write and read sets.
 - tuple_cache: size of memory occupied by tuple cache.
 - page_index: size of memory used for storing page indexes.
 - bloom_filter: size of memory used for storing bloom filters.

It also removes box.stat.vinyl().cache, as the size of cache is now
reported under memory.tuple_cache.