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
Merge pull request #139 from cloudant/25944-changes-descending-mm

Make _changes?descending=true aware of maint. mode

BugzID: 25944