Refactor message layer packages (#331)

* 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