MERGE4: get rid of shunning and use only merging (getting rid of shunfests)

If we have FD_ALL plus shunning, the following scenario can happen:

• A network partition with subgroups {A}

  and

  {B,C,D,E,F,G}
• The partition heals
• A gets heartbeats from all members of the 2nd subgroup
• A's FD_ALL.shun will shun all members of the 2nd subgroup !
• And vice versa, this leads to a shunfest and large clusters might never merge back again

SOLUTION:

• Get rid of shunning (GMS.shun is false by default anyway, now also set FD/FD_ALL.shun to false)
• MERGE4 periodically compares discovery results to its view
  (- This might be done a few times)
• Then MERGE4 initiates a merge between all members who have differing views
• Make sure digests get merged correctly (min/max)