Restconf slow to respond when the member is under load

Marking as a clustering bug, because this is not seen in 1node tests.

Seen in BGP suite (if CONTROLLER-1645 does not come first). The URL is currently /restconf/modules, to be removed soon, but I think this may also happen when querying jolokia.

The read has currently a timeout set to 5 seconds, which can be overstepped [0], but previous reads [1] were also taking more than 4 seconds to finish. I see nothing critical in karaf.log considering this is tell-based protocol.

Note that when testing locally, garbage collector was doing small collects only, so pauses under 1 second.

This might be just a less severe version of 8432 (5s as opposed to 30s), or this might be a less severe version of CONTROLLER-1645 when the delay is not long enough to cause the leader movement due to transient dissociation.

If this is not deemed critical, we can change suites to allow for slower responses.

[0] https://logs.opendaylight.org/releng/jenkins092/bgpcep-csit-3node-periodic-bgpclustering-only-carbon/264/archives/log.html.gz#s1-s9-t8-k2-k3-k7-k2-k1-k6-k1-k1-k1-k1-k1-k2-k1-k1-k2-k4-k2-k1-k6-k3-k1-k2-k1-k1-k3-k3-k1
[1] https://logs.opendaylight.org/releng/jenkins092/bgpcep-csit-3node-periodic-bgpclustering-only-carbon/264/archives/log.html.gz#s1-s9-t8-k2-k3-k7-k2-k1-k6-k1-k1-k1-k1-k1-k2-k1-k1-k2-k3-k2-k1-k6-k3-k1-k2-k1-k1-k3-k3-k1