Add / remove server (dynamic membership)

[belaban]

Currently RAFT.majority defines the majority needed for elections and log commits. This requires the operator to always start the same set of servers. It is easy to start A,B,C, append and commit some changes to the log, then stop them and start D,E,F and make some other changes, overwriting the previous ones and violating Leader Completeness.
We need to define a static membership, e.g. {A,B,C} in the config file, compute the majority from it (2) and use AddServer or RemoveServer to change it dynamically (in running members) and XML editing to change it in the config.
Every member should be identified by a name (logical name), which also names the persistent log. When started, and the name is not in the above list, become read-only (don't participate in elections) or throw an exception.
Adding or removing a server would involve calling AddServer which needs to be acked by a majority of the existing view, and then the config would need to be changed as well.

[belaban]

Implemented dynamic membership changes as described in the Raft dissertation: insertion of internal addServer/removeServer commands into the log.