bgpd: Allow extending peer timeout in rare case

bgpd: Allow extending peer timeout in rare case

Currently the I/O pthread handles incoming/outgoing data
communication with all peers.  There is no attempt at modifying
the hold timers.  It's sole goal is to read/write data to appropriate
channels.  All this data is handled as *events* on the master pthread
in BGP.   The problem is that if the master pthread is extremely busy
then any packet read that would be treated as a keepalive event may
happen after the hold timer pops, due to the way thread events are handled
in lib/thread.c.

In a last gap attempt, if we notice that we have incoming data
to proceses on the input Queue, slightly delay the hold timer.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>