Memtx MVCC memory monitoring

[TarantoolBot]

Root document:
https://www.tarantool.io/en/doc/latest/book/monitoring/
https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_stat/

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Introduce memtx MVCC memory monitoring. One can get it with
box.stat.memtx.tx() method or use index to access a particular
statistic. The statistics format:

txn:
  statements:
    max: 0
    avg: 0
    total: 0
  user:
    max: 0
    avg: 0
    total: 0
  system:
    max: 0
    avg: 0
    total: 0
mvcc:
  trackers:
    max: 0
    avg: 0
    total: 0
  conflicts:
    max: 0
    avg: 0
    total: 0
  tuples:
    tracking:
      stories:
        count: 0
        total: 0
      retained:
        count: 0
        total: 0
    used:
      stories:
        count: 0
        total: 0
      retained:
        count: 0
        total: 0
    read_view:
      stories:
        count: 0
        total: 0
      retained:
        count: 0
        total: 0

Requested by @drewdzzz in tarantool/tarantool@5a99429.

UPDATE(2.11.1): In the initial implementation (version 2.10.0 through 2.11.0 inclusive) box.stat.memtx wasn't callable. This was confusing because other stat entries (box.stat.net, box.stat.vinyl, box.stat.sql) are callable, see tarantool/tarantool#8448. This was reworked in tarantool/tarantool#8471. Now all memtx statistics can be obtained by calling box.stat.memtx() and to get memtx transaction statistics one should use box.stat.memtx().tx. It's still possible to call box.stat.memtx.tx() but this way is deprecated and left only for backward compatibility.