Eliminate global lock on TopicTree

[shoenig]

Right now, every AddSubscription and RemoveSubscription acquire a big nasty lock on the tree. I think this can be avoided by having locks at the node level instead. That way, you only need to W lock on the parent of the node you are removing or adding. All other operations would still be R locks.

I'm not sure if this is worth doing, and right now I just want to get something working.This issue is to remind me to take a closer look someday.