* go/proxyd: Proxy requests as batched RPC

We forward several RPC request objects to upstreams using the JSON-RPC
batch functionality. This should be more efficient than serialized RPC
request proxying the round-trip latency of the remaining request objects
are eliminated.

A new server config, `max_upstream_batch_size`, is introduced to limit
the number of RPC request objects in a single batch request. This is to
avoid overloading upstream as proxyd may accept a large number of
request objects in a single request by having a large `max_body_size_bytes`
config value.

* remove flakes: no more SequencedResponseHandler
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>