node: Add reference to mempool in NodeContext

Currently it is an alias to the global ::mempool and should be used as
follows.

* Node code (validation and transaction relay) can use either ::mempool
  or node.mempool, whichever seems a better fit.
* RPC code should use the added convenience getter EnsureMempool, which
  makes sure the mempool exists before use. This prepares the RPC code
  to a future where the mempool might be disabled at runtime or compile
  time.
* Test code should use m_node.mempool directly, as the mempool is always
  initialized for tests.

src/rpc/blockchain.cpp

@@ -15,6 +15,7 @@
 #include <hash.h>
 #include <index/blockfilterindex.h>
 #include <node/coinstats.h>
+#include <node/context.h>
 #include <node/utxo_snapshot.h>
 #include <policy/feerate.h>
 #include <policy/policy.h>
@@ -53,6 +54,15 @@ static Mutex cs_blockchange;
 static std::condition_variable cond_blockchange;
 static CUpdatedBlock latestblock;
 
+CTxMemPool& EnsureMemPool()
+{
+    CHECK_NONFATAL(g_rpc_node);
+    if (!g_rpc_node->mempool) {
+        throw JSONRPCError(RPC_CLIENT_MEMPOOL_DISABLED, "Mempool disabled or instance not found");
+    }
+    return *g_rpc_node->mempool;
+}
+
 /* Calculate the difficulty for a given block index.
  */
 double GetDifficulty(const CBlockIndex* blockindex)