Web session persistent store cleanup with buddy replication

See parent issue for basic problem description. This subtask is for when buddy replication is enabled.

With BR, the backup copies of sessions are always in another area of the JBC tree, not the main tree. If a webapp undeploys, it's area of the main tree is useless. If it restarts, any persistent store data in the main tree is possibly out of date, superseded by data in another cache that has taken ownership of the session. So, the webapp's main tree region should be cleared. If a request comes in for a session owned by the previous version of the webapp (i.e. the session didn't fail over while the webapp was undeployed on the node), it can be gravitated from wherever the backup copy resides.