Allow users to select which node routing strategy to use for writes

idea1:
Allow users to select the node with least latency as primary write node
(mxs-name/node-name/id vs node-latency )
as well as always sending to current leader
as well as always sending to a named node

idea2:
allow user to define /mark which nodes are allowed to be writable (leader, follower, least latency)

Galera nodes do allow read/write to all nodes and the new maxctrl list servers should reflect that unless we've configured routing to node index 0, in which case only 1 of the nodes is writable.

Maxscale read write state for servers should reflect their configuration.