update: use bandwidth metering when cache is enabled

[daytime-em]

We don't need to change the DefaultBandwidthMeter because the DataSource is entirely responsible for reporting its own bandwidth stats. The approach we take instead is to report data transfer only when the DataSource is transferring from a remote data source.

• MuxDataSource never reports its own data transfer stats. Instead it attaches the BandwidthMeter to the DataSources delegated-to, which are responsible for reporting their stats normally.
• RevalidatingDataSource reports its own stats directly only if we have a response body to download (eg, an expired entry)

[daytime-em]

Drafting for now, will revisit when we are not doing as much