Merge #18740: Remove g_rpc_node global

b3f7f375ef refactor: Remove g_rpc_node global (Russell Yanofsky)
ccb5059ee8 scripted-diff: Remove g_rpc_node references (Russell Yanofsky)
6fca33b2ed refactor: Pass NodeContext to RPC and REST methods through util::Ref (Russell Yanofsky)
691c817b34 Add util::Ref class as temporary alternative for c++17 std::any (Russell Yanofsky)

Pull request description:

  This PR removes the `g_rpc_node` global, to get same benefits we see removing other globals and make RPC code more testable, modular, and reusable.

  This uses a hybrid of the approaches suggested in #17548. Instead of using `std::any`, which isn't available in c++11, or `void*`, which isn't type safe, it uses a small new `util::Ref` helper class, which acts like a simplified `std::any` that only holds references, not values.

  Motivation for writing this was to provide an simpler alternative to #18647 by Harris Brakmić (brakmic) which avoids some shortcomings of that PR (https://github.com/bitcoin/bitcoin/pull/18647#issuecomment-617878826)

ACKs for top commit:
  MarcoFalke:
    re-ACK b3f7f375ef, only change is adding back const and more tests 🚾
  ajtowns:
    ACK b3f7f375ef

Tree-SHA512: 56292268a001bdbe34d641db1180c215351503966ff451e55cc96c9137f1d262225d7d7733de9c9da7ce7d7a4b34213a98c2476266b58c89dbbb0f3cb5aa5d70

src/interfaces/node.cpp

@@ -27,6 +27,7 @@
 #include <sync.h>
 #include <txmempool.h>
 #include <ui_interface.h>
+#include <util/ref.h>
 #include <util/system.h>
 #include <util/translation.h>
 #include <validation.h>
@@ -80,7 +81,7 @@ public:
     bool appInitMain() override
     {
         m_context.chain = MakeChain(m_context);
-        return AppInitMain(m_context);
+        return AppInitMain(m_context_ref, m_context);
     }
     void appShutdown() override
     {
@@ -225,7 +226,7 @@ public:
     CFeeRate getDustRelayFee() override { return ::dustRelayFee; }
     UniValue executeRpc(const std::string& command, const UniValue& params, const std::string& uri) override
     {
-        JSONRPCRequest req;
+        JSONRPCRequest req(m_context_ref);
         req.params = params;
         req.strMethod = command;
         req.URI = uri;
@@ -323,6 +324,7 @@ public:
     }
     NodeContext* context() override { return &m_context; }
     NodeContext m_context;
+    util::Ref m_context_ref{m_context};
 };
 
 } // namespace