refactor: implement server FDv2 streaming synchronizer

[beekld]

This is a pretty straightforward adaptation of Java's streaming synchronizer, using the framework we developed for the polling synchronizer. I don't know of anything particular that would be controversial in this change.

There is also a minor change in the foxy client that should fix the flaky test.

I have tested the unit tests with both curl and foxy clients, although the tests don't particularly depend on the http implementation.

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