fixes to voxel server crashes on client shutdown

This will fix a couple issues we saw today.
* possible deadlock when clients are shutting down while those clients are still being sent to
* possible crash in send thread if clients shut down while still being sent to

The fixes are:
* add a non-blocking mode to nodeList::nodeWithUUID() so we can ask for the node by ID without getting into a deadlock
* support for a mutex on the sending thread process and the node data delete, so that basically, you can't start deleting the nodeData while inside of a processing loop.

closes #2428