There is no reason to have an index on just `realm_id` or `remote_id`,
as those values mean nothing outside of the scope of a specific
`server_id`. Remove those never-used single-column indexes from the
two tables that have them.
By contrast, the pair of `server_id` and `remote_id` is quite useful
and specific -- it is a unique pair, and every POST of statistics from
a remote host requires looking up the highest `remote_id` for a given
`server_id`, which (without this index) is otherwise a quite large
scan.
Add a unique constraint, which (in PostgreSQL) is implemented as a
unique index.
