quic: establish a "happened-before" relationship between stream write and read

This ensures that our race detector will not get tripped off in tests
where we do something like the following:

1. Client writes to a shared variable.
2. Client sends a request to a server.
3. Server handler reads the shared variable.

We are technically over-synchronizing right now by always using the
address of quicSync when calling runtime.RaceAcquire and
runtime.RaceReleaseMerge. However, that is fine for now since this
mostly matters just for testing, and creating a mapping of stream ID to
addressess are rather unclean.

For golang/go#78737

Change-Id: Id65964ad3aa0b2e998e8faf11bfda3c76a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/777940
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>