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/test_bitcoin.cpp

@@ -17,6 +17,8 @@
 #include "txdb.h"
 #include "txmempool.h"
 #include "ui_interface.h"
+#include "rpc/server.h"
+#include "rpc/register.h"
 #ifdef ENABLE_WALLET
 #include "wallet/db.h"
 #include "wallet/wallet.h"
@@ -53,9 +55,12 @@ BasicTestingSetup::~BasicTestingSetup()
 TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(chainName)
 {
     const CChainParams& chainparams = Params();
+        // Ideally we'd move all the RPC tests to the functional testing framework
+        // instead of unit tests, but for now we need these here.
+        RegisterAllCoreRPCCommands(tableRPC);
 #ifdef ENABLE_WALLET
         bitdb.MakeMock();
-        walletRegisterRPCCommands();
+        RegisterWalletRPCCommands(tableRPC);
 #endif
         ClearDatadirCache();
         pathTemp = GetTempPath() / strprintf("test_bitcoin_%lu_%i", (unsigned long)GetTime(), (int)(GetRand(100000)));