Fix issue #3352125: Crash after clearing search query

Don't deliver data arriving from a Loader after the application
has moved on to a new Loader.

Also throttle the number of Loaders will have actively running
for a particular ID.

Oh, and documentation.  Documentation is good.

Change-Id: I5a5cfdbb8c9bfb320ffc014f99e37fe7e3001382