bgpd: Implement group-overrides for peer timers

bgpd: Implement group-overrides for peer timers

This commit implements BGP peer-group overrides for the timer flags,
which control the value of the hold, keepalive, advertisement-interval
and connect connect timers. It was kept separated on purpose as the
whole timer implementation is quite complex and merging this commit
together with with the other flag implementations did not seem right.

Basically three new peer flags were introduced, namely
*PEER_FLAG_ROUTEADV*, *PEER_FLAG_TIMER* and *PEER_FLAG_TIMER_CONNECT*.
The overrides work exactly the same way as they did before, but
introducing these flags made a few conditionals simpler as they no
longer had to compare internal data structures against eachother.

Last but not least, the test suite has been adjusted accordingly to test
the newly implemented flag overrides.

Signed-off-by: Pascal Mathis <mail@pascalmathis.com>