so when there is backpressure in publishing reports, drop shortcut
reports in preference to full reports, and drop old full reports in
preference to new full reports.
Fixes#2738.
We want the middle ground between a small compression size, a fast
compression time and a fast decompression time.
Tests suggest that the default compression level is better than the
maximum compression level: although the reports are 4% bigger and
decompress slower, they compress 33% faster.
See discussion on https://github.com/weaveworks/scope/issues/1457#issuecomment-293288682
* New encoding format:
* Msgpack reports between probe<->app (smaller representation, faster to
encode/decode).
* Still use JSON between app<->UI (try to avoid making javascript deal with
mspack).
The app still suports publishing reports in both gob and JSON, not braking
backwards compatibility.
* Use compile-time generated marshallers/unmarshallers for higher performance. In
order to be able to skip code-generation for certain types, I included
https://github.com/2opremio/go-1/tree/master/codec/codecgen instead of
upstream until https://github.com/ugorji/go/pull/139 is merged.
* Encode/decode intermediate types using github.com/ugorji/go/codec.Selfer
for higher performance and reducing garbage collection (no temporary buffers).
