PGPool::update: optimize removed_snaps comparison when possible

PGPool::update: optimize removed_snaps comparison when possible

In self/unmanaged snaps mode, optimize removed_snaps comparison
for cases where removed_snaps has not changed. This exploits the
fact that remove_unmanaged_snap adds a dummy removed snapshot
to the end of removed_snaps, allowing for inexpensive detection
of changes. In cases where removed_snaps is very large, this
optimization improves performance dramatically.

Signed-off-by: Zac Medico <zmedico@gmail.com>
(cherry picked from commit caf6803b13d9dbd4540da366b018d721fcfc371a)