Bug 1506534 - Collect telemetry to measure how much penalty we will experience with first-party cache isolation, r=mayhemer, data-r=chutten

Whenever a cache entry is accessed during a document load, eTLD+1 of the top level document is added to the entry's metadata. Number of accessing sites is also stored in cache index. So we know how many copies of each entry would we have if we did a first party isolation without data deduplication. The telemetry is sent every time we write 2GB to the cache and then the data is reset. Telemetry report ID is an identifier of the telemetry cycle and it's used to invalidate eTLD+1 hashes in all cache entries.

Differential Revision: https://phabricator.services.mozilla.com/D26425

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extra : moz-landing-system : lando