* replace 'transaction' with 'message' in message layer pkg

* remove more occurrences of 'transaction'

* fixes ReferencedMessageId() returning approverMessageId

* renames ApprovingMessageId to ApproverMessageId

* use ApproverMessageId in deleteFureCone()

* refactors message pkg

* renames payloadId in Message to contentId

* finalize comment on what a Message is

* refactors additional message sub pkgs

* renames 'transaction' to 'message' in gossip plugin

* removes occurrences of 'transaction' in graph plugin

* removes occurrences of 'transaction' in metrics plugin

* refactors dashboard plugin for messages

* removes further occurrences where tx is used instead of msg

* fixes TestMessageRequest in manager_test.go

* go fmt explorer_routes.go

* fix review comments

* fix Errorf order

* throw around bones

* Feat: started reworking output model

* Refactor: refactored some of the model

* Refactor: started to refactor some additional models

* Refactor: started to refactor message layer

* Refactor: still refactoring :/

* Refactor: refactored some more

* Refactor: some error messages are gone YAY

* Refactor: refactor complete

* Feat: started porting the new binary stuff

* Refactor: removed unnecessary folder

* Refactor: cleaned up go.mod files

* Fix: removed objectsdb files

* Feat: added transactionrequester package

* Feat: added new transactionparser as the filter for the tangle

* Feat: started porting the new binary stuff

* Refactor: removed unnecessary folder

* Refactor: cleaned up go.mod files

* Fix: removed objectsdb files

* Feat: added transactionrequester package