This commit makes a few changes to the community history archive
download routine to make it more robust:
1. Prior to this commit, even when there were no archives to be
downloaded, we were still trying to extract messages from archive
data.
2. Logs have been improved as they were sometimes showing confusing
information
3. We now handle interruption of ongoing download + data import much
better in case of multiple magnetlinks being processed in roughly the
same time.
4. We now keep track of which archive has been successfully imported
into the database. Without this, Status would consider any downloaded
archives as "done" even though they haven't actually been imported
into the database yet. This way Status should be able to pick up its
work were it left of the last time, in case a user closes the app, or
another magnetlink interrupts the ongoing process.
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