bgpd: restore packet input limit

Unfortunately, batching input processing severely impacts BGP initial
convergence times. As a consequence of the way update-groups were
implemented, advancing the state of the routing table based on prefixes
learned from one peer prior to all (or at least most) peers establishing
connections will cause us to start generating outbound UPDATEs, which is
a very expensive operation at present. This intensive processing starves
out bgp_accept(), delaying connection of additional peers. When
additional peers do connect the problem gets worse and worse, yielding
approximately exponential growth in convergence time dependent on both
peering and prefix counts. This behavior is present pre-multithreading
as well, but batched input exacerbates it.

Round-robin input processing marginally harms convergence times for
small topologies but should allow much larger topologies to function
within reasonable performance thresholds.

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

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