GMS: join and immediate leave of coordinator doesn't install new view

When we have coord A and B, the following can happen:

• A joins: view=A|0
• B joins: view=A|1
• A leaves, view B|2 should be multicast by A to B
  --> B|2 is never received by B. B has to wait until failure detection detects that A is absent and installs a new view
  --> This is not incorrect, but takes more time than A installing the last view B|2 before it leaves

The reason is that CoordGmsImpl.leave() sets leaving to true, which changes the impl from CoordGmsImpl to ClientGmsImpl. However, the latter doesn't handle the LEAVE request.

SOLUTION:

• Instead of using field leaving, use a (stack-based) variable is_leaving in handleMembershipChange().