Turn on Eliot logging on storage servers

Improved Tahoe-LAFS systemd unit ExecStartPre

This removes the code that might delete a Tahoe-LAFS nodedir and replaces it
with code that will try to save an unexpected copy of it out of the way
instead.

Update to PaymentServer with delayed redemption response

Raise the Tahoe-LAFS storage server file descriptor limit

Somewhat improve the SSH configuration for issuer and storage servers

And test for (kind of) SSH availability

nixpkgs/morph version handling improvements

this means we don't have to download it every time

Maybe I should be wasting my time learning about Hydra instead ...