Provide a non-interactive mode for maintenance

When adding or replacing nodes in a cluster we often don't want them to
be participating in read/write, view, or changes requests. This just
rejects all RPC calls that have an IO class of interactive.

BugzId: 18955
Fix latent bug in get_all_security

Accidentally calling maybe_finish_set instead of the appropriate
maybe_finish_get.

BugzId: 18955
Allow io_priority for design_docs/1

Previously there was no way to specify an io_priority during calls to
fabric:design_docs/1.

BugzId: 18955
Silence errors when getting security objects

Just drop the message and carry on as usual.

BugzId: 18955
Silence notifications on non-interactive nodes

Db update listeners on non-interactive nodes should not send updates
about their databases being updated. While theoretically harmless this
could end up causing some rather ugly behavior when we get update
notifications often but no changes are sent.

BugzId: 18955
Enhance maintenance_mode instead of creating a new mode

BugzID: 18955
Merge pull request #86 from cloudant/18955-non-interactive-mode

BugzID: 18955