MERGE3: a dead member as merge leader will never trigger a merge

When we have member(s) which have a view with a dead member as coordinator, when the dead member becomes part of the subgroup coordinators and happens to be chosen as merge leader, then a merge will never ensue.

Example:

• Member 2 (with view 2|5) was the previous coordinator and left the cluster, installing view 3|6 before stopping
  • View 2|5={2,3,4,5,6,7}; view 3|6={3,4,5,6,7}
• Member 7 didn't get view 3|6 and still has view 2|5
• Everybody else has view 3|6
• MERGE3 gets the following views:
  • 2|5: 7 // member 7 has this view
  • 3|6: 3,4,5,6 // members 3,4,5 and 6 have this view
• 2 and 3 are added to a sorted set and the first member of the set (2) is chose as merge leader. 3 doesn't take any action, as it notices it won't be the merge leader
  • The reason 2 was first in the sorted set is that (possibly by coincidence) its UUID is lower than that of 3. If this wasn't the case, 3 would be merge leader and start (and successfully complete) a merge. However, with dead member 2 being picked as merge leader, a merge will never be triggered!