bgpd: move update group processing to main thread

bgpd: move update group processing to main thread

Prior to this change, packets generated for update groups were taken off
of the (independent) buffer for the update group, reformatted for the
specific peer under question and sent off inline with bgp_write(). Since
the operations of this code path can include the merging and pruning of
subgroups and are too large to safely synchronize, this change moves
that logic to execute after each tick of the write thread.

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