closes#3024
Don't use pages from the cache unless we hold an exclusive write lock,
because a page could be updated by a writer in-memory at any point
before we backfill it.
Clear the WAL tag in other areas to prevent any stale tags. Also, we
will just snapshot the page when we determine that it's eligible, and
pay a memcpy instead of the read from disk, but this further prevents
any in-memory changes to the page/TOCTOU issues, and we also assert that
it's still eligible after we copy it to a new buffer.
Reviewed-by: Jussi Saurio <jussi.saurio@gmail.com>
Closes#3036
16a3410934