bgpd: put BGP keepalives in a pthread

bgpd: put BGP keepalives in a pthread

This patch, in tandem with moving packet writes into a dedicated kernel
thread, fixes session flaps caused by long-running internal operations
starving the (old) userspace write thread.

BGP keepalives are now produced by a kernel thread and placed onto the
peer's output queue. These are then consumed by the write thread. Both
of these tasks are concurrent with the rest of bgpd, obviating the
session flaps described above.

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