[w3c/ServiceWorker] consider how Client API should behave with bfcache (#1038)

In Firefox we have a back-forward cache (bfcache).  When in effect you get `pageshow` and `pagehide` events with the `event.persisted` attribute set to true.  I believe safari has this mechanism as well.

How should we handle the `Client` for windows that go into and out of the bfcache?

My current plan is:

1. Environments in the bfcache will not be exposed to `Clients.matchAll()`.
2. An environment leaving the bfcache will retain its original client ID.
3. Calls to `postMessage()` while the environment is in the bfcache will be queued and delivered if the page comes out of bfcache.

The `postMessage()` thing might be too expensive memory-wise, but I'd like to try it.

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