Merge topic branch t/rm_sha1 into next

The series first drops the para_upgrade_db executable because it is no longer
needed. This removes the penultimate user of the sha1 infrastructure. The
remaining user is the handshake between para_client and para_server.

Since paraslash-0.7.0, client and server employ sha256 by default, but fall
back to sha1 in order to stay compatible to old 0.6.x versions. This series
instructs server and client to always employ sha256 as the hash algorithm,
breaking compatibility with 0.6.x clients. As a result, the sha1 hash algorithm
is no longer used anywhere, so the corresponding infrastructure is no longer
needed. This gets rid of a fair amount of code.

<!--

- The para_upgrade_db executable to convert old paraslash databases has
been removed. paraslash-0.7.0 was released more than three years ago, so
everybody should have upgraded by now.

- Authentication now rejects the weak sha1 hash algorithm, breaking
compatibility with 0.6.x versions. Older 0.7.x clients are still compatible
to current server versions and current clients can still talk to servers
running older 0.7.x versions.

-->

* refs/heads/t/rm_sha1:
  Rename hash2 -> hash.
  Remove sha1.
  Remove para_upgrade_db.
  aft: Compare the full hash, not just the first 20 bytes.