add way to listen for dirty regions

[raisjn]

external clients should be able to listen for dirty region updates. this task is to add some IPC mechanism or otherwise to the server that lets a client subscribe to receive dirty region updates. this will allow VNC client to get their speed upgrades.

one question is: do we allow multiple clients to subscribe? or is it ok to have just a FIFO (for example) and require duplexing to happen somewhere downstream.