This saves memory and CPU time encoding and decoding.
We assume that no probes that old are still running, and we don't need
to reconstruct the data for saved historical reports because controls
are not relevant to historical nodes.
Previously the only roundtrip test was for an empty report.
This test has fake data similar to that found in real reports.
'Metrics' does not round-trip exactly, so a DeepEqual workaround is
added for that.
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
