Should we shutdown the bookie or retry when registration failure

[shoothzj]

about the flaky test #3206

I have figured out why testBookieServerZKExpireBehaviour falky.

It's due to Registration task, bookie will retry to registration when zk session expires. If the registration fail, it will cause the bookie shutdown ExitCode.ZK_REG_FAIL.

@dlg99 @eolivelli @merlimat PTAL

[dlg99]

de-facto doc:

bookkeeper/conf/bk_server.conf

Lines 804 to 809 in 6b03b01

	# ZooKeeper client session timeout in milliseconds
	# Bookie server will exit if it received SESSION_EXPIRED because it
	# was partitioned off from ZooKeeper for more than the session timeout
	# JVM garbage collection, disk I/O will cause SESSION_EXPIRED.
	# Increment this value could help avoiding this issue
	zkTimeout=10000

BK will exit on SessionExpiredException from zk client; it is hard to recover from it (have to re-create the zk client + redo all initialization steps of all subcomponents) so better to exit and let k8s/supervisord restart the service.