bgpd: update pthreads to use lib changes

bgpd: update pthreads to use lib changes

Use the new threading facilities provided in lib/ to streamline the
threads used in bgpd. In particular, all of the lifecycle code has been
removed from the I/O thread and replaced with the default loop. Did not
do the same to the keepalives thread as it is much smaller (doesn't need
the event system).

Also cleaned up some comments to match the style guide.

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