Fix race condition in transaction log management

There was a narrow window in which an I/O operation could be initiated for the wrong page, leading to an Assert failure or data corruption.

/contrib/ltree fixes (Teodor)

Fix longstanding planning error for outer joins

This bug sometimes caused a bogus error “RIGHT JOIN is only supported with merge-joinable join conditions”.

Prevent core dump in pg_autovacuum when a table has been dropped