bgpd: batched i/o

bgpd: batched i/o

Instead of reading a packet header and the rest of the packet in two
separate i/o cycles, instead read a chunk of data at one time and then
parse as many packets as possible out of the chunk.

Also changes bgp_packet.c to batch process packets.

To avoid thrashing on useless mutex locks, the scheduling call for
bgp_process_packet has been changed to always succeed at the cost of no
longer being cancel-able. In this case this is acceptable; following the
pattern of other event-based callbacks, an additional check in
bgp_process_packet to ignore stray events is sufficient. Before deleting
the peer all events are cleared which provides the requisite ordering.

XXX: chunk hardcoded to 5, should use something similar to wpkt_quanta

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