This patch fixes several NullPointerExceptions that were observed in an
cluster when running under high load and latency, including related
issues in the BOSH connector and hazelcast plugin. Some related logging
improvements are also included.

Some of the NPEs were generated during the startup phase before the
server was fully initialized. These have been corrected by checking
appropriate "ready state" indicators.

In general, sessions hosted in remote cluster nodes that cannot be
addressed due to timeouts during remote task execution should be
considered temporarily "unavailable" to the cluster member that
initiated the call.