Use stronger types for transaction marker payloads and activity details

Transaction markers have a payload of type `Document` [here](https://github.com/Graylog2/graylog2-server/blob/ff189e3589b833e3d921b7711527dd5ac8824865/graylog2-server/src/main/java/org/graylog/collectors/db/TransactionMarker.java#L44).

Activity details reflect that by using `Map` [here](https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog/collectors/rest/RecentActivityResponse.java#L35)

We usually try to avoid that and use proper Jackson subtypes with type discriminators to model this.