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Add support for multi-process subscription state change notifications (#7862)

As with the other multi-process notifications, the core idea here is to
eliminate the in-memory state and produce notifications based entirely on the
current state of the Realm file.

SubscriptionStore::update_state() has been replaced with separate functions for
the specific legal state transitions, which also take a write transaction as a
parameter. These functions are called by PendingBootstrapStore inside the same
write transaction as the bootstrap updates which changed the subscription
state. This is both a minor performance optimization (due to fewer writes) and
eliminates a brief window between the two writes where the Realm file was in an
inconsistent state.

There's a minor functional change here: previously old subscription sets were
superseded when the new one reached the Completed state, and now they are
superseded on AwaitingMark. This aligns it with when the new subscription set
becomes the one which is returned by get_active().

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