Favoring mozAlarms over request-sync, and the Gaia email app

[James Burke]

For v2.5 and master, the email app is changing over to using mozAlarms
for background sync requests, instead of using request-sync[1]. Users
of the email app should not be disrupted by the change: there is an
auto upgrade mechanism in place, it just misses one sync, but will
auto sync on the desired interval after that upgrade pass.

We also suggest apps for the near future and for apps already
targeting v2.x FxOS versions to use mozAlarms directly instead of
request-sync. That could change once Service Workers are the
background sync entry point.

The v2.2 email app uses request-sync, so if there are consumers of the
v2.2-related branches that want to uplift this change, it will require
some custom work. Please communicate with us in the bug mentioned
below or contact Dylan Oliver (dol...@mozilla.com) if you want to
coordinate something for that branch.

This change was done for stability reasons. We were getting some
unreliable sync results, and while we have been filing bugs and
helping trace issues, it looks like more resources need to be applied
for request-sync, likely with a re-evaluation of what it needs to do,
and how it interacts with other platform services. We are hopeful that
we can switch back to it during the Service Worker age of entry
points.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1219853