- restore the old behavior of the connection pool replacing itself just

- restore the old behavior of the connection pool replacing itself just
within userland engine.dispose(); as some SQLA tests already failed when the replace step
was removed, due to those conns still being referenced, it's likely this will
create surprises for all those users that incorrectly use dispose()
and it's not really worth dealing with.  This doesn't affect the change
we made for ref: #2985.