schemeshard: measure TTableInfo::VerifyConsistency() time (#42201)

Add a cumulative counter (TablePartitionsConsistencyCheckTimeNs)
accumulating the nanoseconds spent in TTableInfo::VerifyConsistency(),
so the O(N) check's production cost is observable.

VerifyConsistency: time the body with THPTimer into the new mutable
LastVerifyConsistencyTime. Gate the whole check behind the new
EnableTablePartitionsConsistencyCheck feature flag (default on); when
off, return early and reset LastVerifyConsistencyTime to 0.

The partitioning calls that run the check — SetPartitioning,
MovePartitioning, CopyPartitioning, ApplySplitMerge, and move_table —
increment the counter by LastVerifyConsistencyTime afterwards.

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