Enable capability to disable a single invoker for maintenance

In an OpenWhisk deployment with multiple invokers, there may be an occasion to perform maintenance on a single invoker machine without wanting to bring down the entire OpenWhisk system.

Steps might look something like:
- mark invoker as going-offline
- controller no longer sends work to this invoker
- invoker waits for current activations to complete
- existing invoker queued action requests are either run/completed or re-queued to another invoker.. need discussion
- invoker marks itself as offline
- maintenance can be performed
- invoker restart does not cause invoker to automatically become online.
- mark invoker as being online

related issues
#2678

Start by reviewing the controller and invoker lifecycle described in the issue, along with related issue #2678. Clarify the behavior for queued requests and restart state before identifying implementation entry points. Done means a single invoker can be taken offline for maintenance without disrupting the deployment, then explicitly returned online.

Written by the indexing model from the issue text.