Triggers should activate actions via internal interface

Currently a trigger causes an action activation through an external interface - but we should use an internal interface.

- [ ] refactor entitlement check to authorize action
- [ ] create internal interface for activation a trigger (useful to surface web triggers)
- [ ] replace http post request for internal interface
- [ ] restoring ability to track causality

Also consider as part of this some long needed changes to handing of triggers:
- [ ] persistence of trigger and/or rule activations https://github.com/apache/incubator-openwhisk/issues/512
- [ ] logging activation ids (or failures)

Start with the issue checklist and the linked issue #512, then trace the existing trigger-to-action activation path in the OpenWhisk codebase. Done means the listed interface, authorization, HTTP replacement, causality, persistence, and activation logging concerns are resolved or explicitly scoped.

Written by the indexing model from the issue text.