`Receptor` and `Emitter` concepts and related renamings

[alexander-yevsyukov]

This PR renames HandlerMethod to Receptor and performs other naming improvements related to this change. Also, methods that produce messages are now called Emitters.

So the convention is this:

• Receiver, Producer, Subscriber, Assignee, and Reactor are objects that handle messages.
• Receptor and Emitter are methods of corresponding objects.

Other notable renamings:

• CommandAssignee_Everything_ is renamed to be just Assignee_Everything_. We don't have other Assignees other than those that deal with commands.
• CommandReactionMethod -> CommandingReaction.

There are more renamings introduced by this PR. Those above are given to describe the general schema of name changes.

[codecov[bot]]

Codecov Report

Merging #1484 (98e239b) into master (6b423a1) will decrease coverage by 0.03%. The diff coverage is 90.51%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1484 +/- ## ============================================ - Coverage 91.72% 91.69% -0.04% + Complexity 4969 4967 -2 ============================================ Files 627 627 Lines 15235 15239 +4 Branches 873 875 +2 ============================================ - Hits 13975 13974 -1 - Misses 956 961 +5 Partials 304 304 ```

[alexander-yevsyukov]

@armiol, PTAL.