Adds Iterables.toSet()
Removes unnecessary intermediate variable in ClusterConfiguration.removeElected
Makes wording on instance failure in HeartbeatContext consistent
Reenables testing of HeartbeatContext. Adds test for a bug and a fix for it.

A problem which is sometimes encountered is that an instance may enter the cluster,
 suspect an instance, send that suspicion everywhere and then exit. All remaining instances
 however will keep the suspicion in their state, which leads to errors, such as instances
 being marked as failed while they are not. This is now tested against and fixed.
A further improvement on this should be the purging of stale entries on instance exit,
 but that can come later.
Ensures instances ignore suspicions of themselves

It is possible for an instance to receive a suspicions set from another instance
 that contains the receiving instance. This however should be ignored, as receiving
 a suspicions message is proof of connectivity. Moreover, it is possible if the
 suspicions is the last message sent from an instance for the receiver to
 permanently suspect itself. This commit adds a simple filtering operation
 that removes the receiver from any received suspicions set.
Adds some tests for this.
Also fixes some formatting.
Ensures that master always sends masterIsAvailable in response to masterIsElected

It is possible for a cluster to deadlock if for some reason (usually very long GC or
 network connectivity issues) an instance goes to pending but the master remains elected.
 When the instance rejoins, it will ask for election, it will receive the masterIsElected
 event but it will also require a masterIsAvailable, as it doesn't receive a snapshot.
 This commit fixes this problem by ensuring that the current master will always respond
 with a masterIsAvailable when it receives a masterIsElected for itself.
Introduces unit tests for the above.
Adds extra cluster condition in HaLoggingIT test
Adds basic functionality testing for HighAvailabilityMemberStateMachine
Merge pull request #1440 from digitalstain/1.9-maint

A set of fixes around heartbeats and state transitioning in HA