memtx: use MemtxAllocator stats for box.info.memory

We need MemtxAllocator stats for reporting read view memory usage, see
https://github.com/tarantool/tarantool-ee/issues/143. Let's also use
them for reporting data memory usage in box.info.memory() for
consistency and add a test.

It looks more correct that using allocator stats for reporting data
memory usage because box.info.memory() is supposed to show high level
stats. For low level allocator stats we have box.slab.info().

Note that a memtx tuple may be held by tuple_bless so we have to reset
it by creating a new tuple in Lua before checking memory usage.

NO_DOC=refactoring
NO_CHANGELOG=refactoring