bgpd: speed up "no-hit" withdraws for routeservers

bgpd: speed up "no-hit" withdraws for routeservers

This accelerates handling of incoming Withdraw messages for routes that
don't exist in the table to begin with.  Cisco IOS 12.4(24)T4 has a bug
in this regard - it sends withdraws instead of doing nothing for
prefixes that are filtered.

Pulling up the adj_in removal in Quagga should have no ill effect, but
we can avoid the costly iteration over all rsclients if there was no
adj_in entry.

Performance impact of this change on routeserver with 3 buggy peers,
startup/sync time:

before patch:  143.12 seconds (user cpu)
after patch:     7.01 seconds (user cpu)

Many thanks to Nick Hilliard & INEX for providing real-world test data!

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Acked-by: Paul Jakma <paul@jakma.org>