vinyl: fix duplicate multikey stmt accounting with deferred deletes

`vy_mem_insert()` and `vy_mem_insert_upsert()` increment the row count
statistic of `vy_mem` only if no statement is replaced, which is
correct, while `vy_lsm_commit()` increments the row count of `vy_lsm`
unconditionally. As a result, `vy_lsm` may report a non-zero statement
count (via `index.stat()` or `index.len()`) after a dump. This may
happen only with a non-unique multikey index, when the statement has
duplicates in the indexed array, and only if the `deferred_deletes`
option is enabled, because otherwise we drop duplicates when we form
the transaction write set, see `vy_tx_set()`. With `deferred_deletes`,
we may create a `txv` for each multikey entry at the time when we
prepare to commit the transaction, see `vy_tx_handle_deferred_delete()`.

Another problem is that `vy_mem_rollback_stmt()` always decrements
the row count, even if it didn't find the rolled back statement in
the tree. As a result, if the transaction with duplicate multikey
entries is rolled back on WAL error, we'll decrement the row count
of `vy_mem` more times than necessary.

To fix this issue, let's make the `vy_mem` methods update the in-memory
statistic of `vy_lsm`. This way they should always stay in-sync. Also,
we make `vy_mem_rollback_stmt()` skip updating the statistics in case
the rolled back statement isn't present in the tree.

This issue results in `vinyl-luatest/select_consistency_test.lua`
flakiness when checking `index.len()` after compaction. Let's make
the test more thorough and also check that `index.len()` equals
`index.count()`.

Closes #10751
Part of #10752

NO_DOC=bug fix

(cherry picked from commit e8810c555d4e6ba56e6c798e04216aa11efb5304)