Currently, MVCCIterator mandates that HasPointAndRange() must return a consistent value with true. However, this means that pebbleIterator.HasPointAndRange() needs to call through to Valid() to do additional checks (e.g. that we haven't moved into the lock table), but most times the caller has already called Valid(). This additional validation can be expensive, and HasPointAndRange() has been seen to have a non-negligible cost.

We should change the MVCCIterator contract to require Valid() to be called before HasPointAndRange(), to avoid this cost.

Jira issue: CRDB-17273

Epic CRDB-2624