This allows for the backend to more efficient route traffic.  Normally
we'd extract this from the request, but location is not part of the
write identifier.

With recent multiplex refactors, call options were not being propagated
properly for non-multiplex writers as we formerly created a
pool-per-writer.

This allows the router to build exclusive connections using the writers
settings, namely overrides to flow control and call options propagated
to the underlying AppendRows RPC.

Towards: googleapis#7103