Make _changes?descending=true aware of maint. mode

The `_changes` coordinator was using a one-off thing to get the start sequence for a database which did not provide any error handling at all. This patch replaces that one-off with a call to `get_db_info/1`, which does include all the normal error handling.

A future enhancement could teach the coordinator to understand a "sequence" like `eof` that would cause all the workers to automatically seek to the end, but I wanted to stay conservative here in order to land this for the next release.

BugzID: 25944