bgpd: intelligently adjust coalesce timer

bgpd: intelligently adjust coalesce timer

The subgroup coalesce timer controls how long updates to a particular
subgroup are delayed in order to allow additional peers to join the
subgroup. Presently the timer value is 200 ms. Increase it to 1 second
and adjust up as peers are configured, with an upper cap at 10s.

This cuts convergence time by a factor of 3 at large scale (300+ peers,
1000+ prefixes per peer).

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>