Allow the addReplace action to replace existing nodes while keeping the same node id.

This allows transparent swapping of a synth (e.g. updating synthdefs with a new version) while ensuring all references to the node remain valid. Allow this on Groups for consistancy, though it's functionally identical to freeing all of the Group's children.
Fix whitespace.
Merge pull request #1130 from scztt/topic/replacenode

server: addReplace: replace existing nodes while keeping the same node id.