Clean up HTTP and JSON-RPC parsing code
http_parserpp may be employed in json-rpc-test-server.
backend_co_on_data etc. can then be deduplicated.
In the bi-directional mode the shell may act as a server and JSON-RPC parsing code can similarly be shared between the two programs. We need to be able to distinguish request from responses and branch accordingly. Think about detecting probable JSON-RPC 1.0 requests and responses--maybe just look at object field names.
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