bgpd: do not send keepalives when KA timer is 0

bgpd: do not send keepalives when KA timer is 0

RFC4271 specifies behavior when the hold timer is sent to zero - we
should not send keepalives or run a hold timer. But FRR, and other
vendors, allow the keepalive timer to be set to zero with a nonzero hold
timer. In this case we were sending keepalives constantly and maxing out
a pthread to do so. Instead behave similarly to other vendors and do not
send keepalives.

Unsure what the utility of this is, but blasting keepalives is
definitely the wrong thing to do.

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