Include changes for AfterUpdateEvents and emit WebSocket events

AfterUpdateEvents now have a new Map property which contains all changes. In constrast to the previously existing changes Map for events of patch operation this also contains implicit changes which have not been requested (e.g. the revision).

A WebSocket event is now emitted for update operations which contains a Map of all changed properties.

STOMP event structure

Topic: <room-id>.<entity-type>.<entity-id>.changes.stream Payload structure:

{
  "changedEntityProperty1": "changedEntityValue1",
  "changedEntityProperty2": "changedEntityValue2",
  "revision": "changedEntityRevision"
}

Closes: #104 (closed)
Depends-On: !370 (merged)
Supersedes: !360 (closed)