* 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: refactored message + started to add tests

* Refactor: go mod tidy

* Refactor: continued to refactor message

* Feat: updated to last hive.go + added Signature() methid

* Feat: go mod tidy

* Feat: added mutex to the signature write in Bytes()

* Fix: fixed Signature method

* Feat: refactored message methods

* Feat: refactored output

* Fix: fixed signature mismatch in tangle factory methods

* Refactor: go mod tidy

* Feat: added solditification logic to value tangle

* Feat: fixed some code related to objectstorage factories

* 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