bgpd: atomize write-quanta, add read-quanta

bgpd: atomize write-quanta, add read-quanta

bgpd supports setting a write-quanta that serves as a hint on how many
packets to write per I/O cycle. Now that input is buffered, it makes
sense to add the equivalent parameter for how many packets are processed
per cycle. This is *not* how many packets are read off the wire per I/O
cycle; rather it is how many packets are processed from the input buffer
in a given cycle after having been read off the wire and sanitized.

Since these values must be used from multiple threads, they have also
been made atomic.

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