Use PipelinePacks inside the MGI infrastructure rather than having a separate
messageHolder struct that serves essentially the same purpose.
Merge branch 'dev' into features/no-messageHolder

Conflicts:
	pipeline/pipeline_runner.go
	pipeline/sandbox_filter.go
Merge branch 'dev' into features/no-messageHolder
Merge pull request #93 from mozilla-services/features/no-messageHolder

Features/no message holder