rpc: Register calls where they are defined

Split out methods to every module, apart from 'help' and 'stop' which
are implemented in rpcserver.cpp itself.

- This makes it easier to add or remove RPC commands - no longer everything that includes
    rpcserver.h has to be rebuilt when there's a change there.
- Cleans up `rpc/server.h` by getting rid of the huge cluttered list of function definitions.
- Removes most of the bitcoin-specific code from rpcserver.cpp and .h.

Continues #7307 for the non-wallet.

src/test/rpc_tests.cpp

@@ -36,7 +36,7 @@ UniValue CallRPC(string args)
     string strMethod = vArgs[0];
     vArgs.erase(vArgs.begin());
     UniValue params = RPCConvertValues(strMethod, vArgs);
-
+    BOOST_CHECK(tableRPC[strMethod]);
     rpcfn_type method = tableRPC[strMethod]->actor;
     try {
         UniValue result = (*method)(params, false);