Loggregator should use go-loggregator for generated envelopes

[poy]

Loggregator using go-loggregator would enable a few nice things:

1. Not having generated code twice. Not a big deal, but still nice.
2. Loggregator-Release would show what version of go-loggregator the given version of Loggregator was built against.
3. Loggregator could "dog-food" the ingress and egress clients.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/154245522

The labels on this github issue will be updated when the story is started.