Concurrent addition of multiple members is not correct

[belaban]

DynamicMembershipTest.testAddServerSimultaneously() is not correct: majority in a RequestTable.Entry is cached (per call, possibly with multiple entries being added to the request-table), and the number of votes required for a majority is not correct.
Perhaps use raft.majority() dynamically?

[belaban]

Still fails... investigating

[belaban]

The issue is that - when adding D and E almost simultaniously to {A,B,C} - D requires a majority of 2 (of {A,B,C}), but E should require 3 (out of {A,B,C,D}). However, at the time, E is being added, D might not yet have been committed (changing the membership from {A,B,C} -> {A,B,C,D}), therefore E might still see the membership as {A,B,C}, and therefore falsely use a majority of 2 rather than 3.

[belaban]

Solution: the leader chains all add-server and remove-server requests calling setAsync() and chaining the returned CompletionStage with all elements of the queue (removing them). This way, a server is added or removed only when the previous request has been completed (committed).

[belaban]

Code:

protected CompletableFuture<byte[]> changeMembers(String name, InternalCommand.Type type) throws Exception {
    InternalCommand cmd=new InternalCommand(type, name);
    byte[] buf=Util.streamableToByteBuffer(cmd);

    // only add/remove one server at a time (https://github.com/belaban/jgroups-raft/issues/175)
    return add_server_future=add_server_future
      .thenCompose(s -> setAsync(buf, 0, buf.length, true, null));
}