memtx: add MemtxAllocator statistics

We account three metrics:

 - used_total - total size of allocated memory
 - used_rv - size of memory held for read views
 - used_gc - size of memory freed on demand

We'll need them to report memtx read view statistics.

While we are at it, switch the test to TAP and drop the result file.

Needed for https://github.com/tarantool/tarantool-ee/issues/143

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