Makes field final in PaxosClusterMemberEvents.ClusterMemberSnapshot

Also fixes some formatting in PCME
Fixes cluster joining process for Ha backup clients

There were two problems with HaBackupProvider, both on cluster
 joining. One was that it depended on snapshots to discover the
 state of the cluster, while since 1.9.6 this has changed and joining
 instances must as for elections. The other was that it passed in a null
 for a SnapshotFilter, which caused NPEs and did not let the availability
 messages to be processed. Both problems are now solved.
Changes ignore reason for BackupHaIT test

Before it was a pending backport. Unfortunately, there appears to be
 some flakyness in the test, which points to a bug. Needs investigation.