txn: collect txn allocation statistics

Since transaction can allocate objects on its own, we need to track it
to collect memory statistics of memtx txm. Txn's use only region
allocator so this patch introduces tx_region methods - txns should use
it instead of region methods to collect allocation statistics.

Part of #6150

NO_DOC=no visible changes
NO_CHANGELOG=no visible changes
NO_TEST=already covered