[patch] Cache invalidation for I/O Workers

This solves BUG #19622, from pgsql-bugs [1]

With io_method=worker I/O worker processes retain open file descriptors on
relation files that have been dropped.

When dropping a relation, an invalidation message is posted, backends handle it
this patch implements the same pattern in I/O worker, and prevents holding
invalid relations.

That said,

This issue doesn't have observable consequences to the user, and is
currently bound
by `max_files_per_process` (default 1000), after which it forces
closing the least
recently used files, if that parameter does its job, we wouldn't have a crash.

In the theoretical scenario of a relation number being reused it could
attempt to read
from a stale file.

https://www.postgresql.org/message-id/19622-639a4ba94c5a53d7%40postgresql.org