Bulk sending messages to things/features via search (RQL) filter

[thjaeckle]

Similar to #1419 it could be quite easy to formulate and perform a "bulk" message sending API call which:
a) selects a set of targeted things or features
b) sends the same message to all of them

Example:

• toggle all of the "switchable" features currently being "on" of things in a home ("leaving home" use case)

Example HTTP API

POST /api/2/bulk/features/inbox/messages/toggle?filter=
  and(
    eq(/attributes/homeName,'My-At-Home'),
    eq(definition,'https://eclipse.github.io/ditto-examples/wot/models/switchable-1.0.0.tm.jsonld'),
    eq(properties/on,true)
  )

This API would need a way to search on features and not on things.
And the filter would have to be able to address both things payload (maybe in an absolute way like /attributes/homeName) and relative feature payload (maybe in a relative way like definition or properties/on).