Move RPC-related sources to src/rpc/ #5705

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I like the idea to move the RPC files, but before we go moving files around I'd like to make sure this is the eventual solution (to not make the mistake of 0.10 to play musical chairs with the file paths :-) ). And also to make sure this doesn't collide with existing efforts.

RPC as we regard it actually consists of multiple components:

• A. RPC server library (rpcserver.cpp) - general, seperated out in #5107
• B. RPC handlers for bitcoind (rpcblockchain.cpp, rpcwallet.cpp, ...)
• C. HTTP server (part of rpcserver.cpp and rpcprotocol.cpp now) - general, separated out in #5677
• D. RPC protocol support (part of rpcprotocol.cpp)
• E. RPC client library (rpcclient.cpp) - only one function that converts linear CLI arguments to requests structure. Used by GUI and bitcoin-cli
• F. HTTP+RPC command-line client (bitcoin-cli.cpp)

There is also:

• G. JSON parsing and serialization - now using spirit, replaced with internal implementation in #4738

Is it too crude to move A-E all to the same directory? To think of a logical separation, one'd want to separate client, server and common. e.g.

• rpc-server: A,B,C
• rpc-client: E (and possibly F, but having utiltiies top-level source in src/ makes sense)
• rpc-common: D,G

(or rpc/server, rpc/client, rpc/common respectively, but I personally don't like having source files in deep directory hierarchies)

Closing this for now, can be revived after other RPC changes as mentioned above are in.