When updating a large number of places, sending runnables to the main thread
for every single one of them whose frecency we update is not conducive to a
responsive UI. This only gets worse once more observers care about these
notifications (e.g. when the library is open).
To avoid this on startup when importing from other browsers, this patch adds
and uses an option to group the frecency notifications. Later patches will
also use the option to avoid other notifications where possible.
MozReview-Commit-ID: D5KqPDu86bo
705481de09