Refactoring

- Upgrading abaaso version dependency
- Changing `compress` dtrace probe to return the Etag instead of data
- Changing `messages.NO_CONTENT`to be `null`
- Refactoring `respond()` to assign an Etag if one is not present based on `url-body` & responding with a `304` when appropriate
- Refactored `server.compressed()` to create the cached version on disk from the result of a non-local (disk) response instead of calling `cache()` which essentially scheduled two compressions