Support starting while offline.

[tomprince]

Currently, when we start the service, we check that the tahoe node is connected to enough servers. (see here). This means that magic-folder can't start until it is online.

In #420 and #395, I want to add some additional checks to the DMDs we are going to use, which requires some network round-trips.

If want to support starting off-line, we could currently just remove the existing connected-enough check. But once #420 and #395 land, we'd need to handle a degraded state, where the upload and download services for each folder aren't created.

[meejah]

Yes, we should handle starting offline. The "connected enough" check doesn't make sense where it is anyway (there's some other ticket about that already) and we already do (and have to) handle the case where we can't currently upload to or download from Tahoe.