Flushing mvcc changes to disk requires serialization. To do so we simply
introduce a lock for pager.end_tx, which will take ownership of flushing
to WAL. Once this is finished we can simply release lock.
When multiple tx writes happen concurrently in mvcc, max frame will be
updated. This new max_frame makes is the point of view of the other
transaction return busy because his current wal snapshot is outdated.
Closes#3059
7fdb116d41