Hands needed: RPC refactoring

[metux]

Describe the idea

Listing several refactoring tasks to be done in the request handlers.
Everybody feel free to get your hands dirty and submit PRs.

• use x_rpcbuf_t for assembling (and byte-swapping) the reply payloads - get rid of individual WriteToClient() calls on little chunks
• write out the header and payload in one shot after everything's finally assembled
• see eg:
• • dix: add X_SEND_REPLY_WITH_RPCBUF() macro xserver#698
• • dix: use x_rpcbuf_t in ProcQueryTree() xserver#679

Already did huge part of it, but lacking the capacity to go through all the code and do the repetitive tasks of fixing up all the individual request handlers.

It should be done because

• marshaling currently is very complicated
• also the buffering and IO
• in the future we want to support pure message based transports (eg. shared memory, infiniband, ...), where we need to collect each packet/message in one block
• gradually move to multi-threading (one thread per client)
• simplify extending the protocol

What are the alternatives?

No response

Additional context

No response

Extra fields

• I have checked the existing New Ideas, RFCs Of The Core Team, Ideas Soon To Be Addressed and Someday Ideas
• I have read the Contributing Guidelines
• I'd like to work on this idea