Added unmarshalling for legacy Loggregator messages.

[#76284328]
Signed-off-by: Johannes Petzold <jpetzold@pivotal.io>
improve test coverage

- go fmt
- clean up obsolete comments

[#76284328]

Signed-off-by: Johannes Petzold <jpetzold@pivotal.io>
add legacy message converter

[#76284328]

Signed-off-by: Trevor Ackerman <tackerman@pivotal.io>