Services to keep track of download counts

This is done in the underlying net_download() function, so service
implementations do not need to do this directly.
Clarify throttling configuration

Rephrases the configuration screen's throttling section to explain what
it is used for (i.e. batch processing and not usual add-on usage).
Pass download count with the miss callback

When a cache miss occurs, wait to call the callback until after the
service run completes; at that point, call the callback and pass the
download count so it can count against the throttling.
Update help documentation for MP3s config

Adds discussion about services requiring multiple downloads for a single
input phrase and updates the screenshot.