Bugdroid today works like this:
* Download bitmaps keeping track of every CL it has ever seen
* Poll gerrit for changes that have been Submitted recently
* Update bitmap to include those changes
* Upload bitmap
* Restart

Instead, it should work like this:
* Configure a plugin like git-numberer
* Listen to RefOperationValidationLister.onRefOperation and OnSubmitValidationListener.preBranchUpdate events

This will also let us do cool things like
* POST to monorail when a change is uploaded as well as when it is landed
* Not have to store any state ever at all thank god
* Delete bugdroid, delete bugdroid's crazy appengine app, delete bugdroid's luci config, never have to worry about configuring bugdroid to watch a new repo ever again
* Expose bug state directly in the PolyGerrit UI if we really want to