Add box.info.memory() to report aggregated memory statistics

This patch adds a new function, box.info.memory(). The functions returns
a table with the following fields:

 - data - size of memory in bytes used for storing user data (i.e.
   tuples) in memtx and in vinyl level 0, without taking into account
   memory fragmentation.

 - index - size of memory in bytes used for indexing user data. This
   includes memtx and vinyl memory tree extents, vinyl page index, vinyl
   bloom filters.

 - cache - size of memory in bytes used for caching user data. Memtx
   doesn't have cache so basically this is the size of the vinyl tuple
   cache.

 - tx - size of memory in bytes used up by active transactions. For
   vinyl it is the total size of all allocated struct txv, struct vy_tx,
   struct vy_read_interval, plus tuples pinned by those objects. For
   memtx it is going to be 0 for now as memtx transaction manager
   shouldn't consume much memory. We may want to account struct txn and
   struct txn_stmt there too in future.

 - net - size of memory in bytes used up by network input and output
   buffers.

 - lua - size of memory used by the Lua runtime.

It is supposed to be used by the admin to get a general knowledge about
what's going on with a particular tarantool instance. For more info, per
subsystem statistics are supposed to be used (e.g. box.info.vinyl()).

Closes #934