Commit 582200a made `ReusableStringStream`'s index reservation thread safe, however it's still accessing the `m_streams` vector outside the lock. This makes it so that the `add` call returns the pointer in addition to the index so that the lock doesn't need to get acquired again until destruction.
The issue with accessing `m_streams` outside the lock is that `add` can call `push_back` on the vector, which might re-allocate. If this re-allocation occurs concurrently with anther thread trying to index into this array, you get UB (typically a null pointer read).
This builds on the existing work to make assertion thread safe,
by adding an extra synchronization point in the holder of
`ReusableStringStream`'s stream instances, as those are used to
build the messages, and finishing the move of message scope holders
to be thread-local.
