The assistant should support upgrading itself.

## non-distro upgrades

When git-annex was installed from this website, the assistant should poll
periodically (once a day or so) to see if there is a new version.
It downloads, over https, a .info file, which contains a serialized data
type containing upgrade information. The url it's downloaded from is
configured by setting `UPGRADE_LOCATION` when building git-annex on the
autobuilders.

When a new version is found, the webapp prompts the user to start the
upgrade. (annex.autoupgrade can be set to true to upgrade w/o prompting.)

The upgrade process is automatic, and rather tricky. The file is downloaded
using git-annex (as a regular key!), and is then unpacked into a new
directory, and the programfile updated to point to it. Then git-annex
restarts itself.

### manifest files

To clean up the old installation, a git-annex.MANIFEST file is looked for
in it, and the files listed, as well as empty directories, are deleted.
I don't want to accidentally delete something I didn't ship!

## restart on upgrade

When git-annex is installed from a proper distribution package, there is no
need for the above. But, the assistant still needs to notice when git-annex
get upgraded, and offer to restart (or automatically restart when
annex.autoupgrade is set).

This is done using the DirWatcher, watching the directory containing the
git-annex binary. Or, in the case of a non-distro install, watching the
directory where eg git-annex.linux/ was unpacked.

When an change is detected, restart.

## multi-daemon upgrades

A single system may have multiple assistant daemons running in different
repositories.

In this case, one daemon should do the non-distro upgrade, and the rest
should notice the upgrade and restart.

I don't want every daemon trying to download the file at once..

Approach: The first new version is installed into a stable directory, based
on its version. So, start the upgrade by making this directory. If upgrade
is already in progress, the directory will already exist. (Remove directory
if upgrade fails.)
