Call set_io_priority/2 before fetching design documents

This change causes nodes that are in maintenance mode to drop view
requests earlier than they would otherwise. Without it, calls to
ddoc_cache:open_doc/3 are being made on nodes that are in maintenance
mode, with what appears to be undesirable results.

BugzID: 27483
Merge pull request #142 from cloudant/27483-check-io-priority-earlier

Call set_io_priority/2 before fetching design documents