achieve update "watershed" for ESR60-based Tor Browser

Once Tor Browser is based on the ESR60 code, because of the switch to xz compression and SHA384-based signing for the MAR files, we will need to have an update "watershed" event. The purpose of this ticket is to track the things we need to do to make the watershed happen.

Let's suppose that for our stable channel the first release that uses the new MAR format is 8.0. This means:

• When an older version of Tor Browser (< 8.0) requests an update, it must receive an old format MAR file that will update it to 8.0. We must keep these MAR files around as long as we care about updating older browsers (probably a long time).
• Newer versions of Tor Browser (>= 8.0) must receive new format MAR files (of course).

Of course we will need to do this sooner for our alpha channel.

My guess is that it will make things easier if we use a new location for the new update requests, e.g., from https://aus1.torproject.org/torbrowser/update_4.

One useful thing to know is that setting MAR_OLD_FORMAT=1 in the environment before running the make_full_update.sh or make_incremental_update.sh script causes the old bzip2 compression to be used when creating the MAR file.