Support lazy failovers in `praefect dataloss`

With the recent failover changes, the output of `praefect dataloss`
is no longer accurate. Previously a repository would have been in
read-only mode if the primary of the repository was outdated. With
lazy failovers in place, it's no longer sufficient to check only
whether the current primary is outdated or not. If the current primary
is outdated, Praefect would immediately switch the repository's primary
on the next request if there is an up to date replica available. This
also means that there is no 'read-only mode' anymore, as we'd simply
failover to an up to date node rather than wait for the current primary
to be brought up to speed. This commit updates the dataloss sub-command
to take the new changes into account:

1. If there is an up to date, available replica for the repository, it's
   considered to be available for both reads and writes.
2. If there are no up to date replicas available, the repository is considered
   unavailable. As it is, Praefect does not distribute writes to outdated
   replicas.
3. To make it easier to determine why a repository is unavailable, 'unavailable'
   is printed next to the storages which are considered to be unavailable by the
   consensus of the Praefect nodes.

Changelog: changed