ODL under load do not send Keepalives quickly enough.

Seen on recent Lithium snapshot builds.

A Sandbox run [0] for new suite (2 iBGP peers, 1 million routes total, continuous GET in IPv4 topology during initial synchronization) has seen one suite fail. Output from play.py shows holdtimer was overstepped, karaf.log shows frequent OperationsLimiter activity.

Previous runs did not see this bug, more data will get available when the suite is merged. Increased number of peers would risk triggering BGPCEP-289.

[0] https://jenkins.opendaylight.org/sandbox/job/bgpcep-csit-1node-periodic-bgp-ingest-only-stable-lithium/3/