Handle CMG-related network messages in CMG thread pool

ClusterManagementGroupManager handles a few messages (including CmgInitMessage and ClusterStateMessage). Currently, the default inbound pool is used for this. But these messages touch storage and their handling might take around 10-20ms (or more), which is a bit too much for the inbound pool.

The handler has to be registered with a pool that will be used for handling the messages. It would be great to reuse some existing pool and not create a new one.