Better feedback during startup

[meejah]

When starting up, magic-folder checks the local database for updates that were "in progress" at the time of the last shutdown.

For example, a LocalSnapshot that has been uploaded and turned into a RemoteSnapshot but not yet linked into the Personal directory. This can take considerable time if there are lots of such snapshots, or if the directory already contains many entries (because tahoe can be a little slow to update very large directories).

A solution here could be to introduce some "status" messages that indicate where the program is in this process. This would allow UI programs to show the user that something is happening (and, e.g., how many more local files are to be checked, etc).

For some concrete numbers: with 35000 local snapshots in a collective with 30,000+ entries, it can take "seconds" to update any that are out-of-date (and thus take quite a while to update everything, depending on how many are out-of-date).