When the mutation compactor has all the rows it needs for a page, it
saves the decision to stop in a member flag: _stop.
For single partition queries, the mutation compactor is kept alive
across pages and so it has a method, start_new_page() to reset its state
for the next page. This method didn't clear the _stop flag. This meant
that the value set at the end of the previous could cause the new page
and subsequently the entire query to be stopped prematurely.
This can happen if the new page starts with a row that is covered by a
higher level tombstone and is completely empty after compaction.
Reset the _stop flag in start_new_page() to prevent this.
This commit also adds a unit test which reproduces the bug.
Fixes: #12361Closes#12384
