Remove the remaining fee estimation globals

This moves the CBlockPolicyEstimator to the NodeContext, which get rids
of two globals and allows us to conditionally create the
CBlockPolicyEstimator (and to remove a circular dep).

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

src/rpc/blockchain.cpp

@@ -17,6 +17,7 @@
 #include <node/coinstats.h>
 #include <node/context.h>
 #include <node/utxo_snapshot.h>
+#include <policy/fees.h>
 #include <policy/feerate.h>
 #include <policy/policy.h>
 #include <policy/rbf.h>
@@ -81,6 +82,15 @@ ChainstateManager& EnsureChainman(const util::Ref& context)
     return *node.chainman;
 }
 
+CBlockPolicyEstimator& EnsureFeeEstimator(const util::Ref& context)
+{
+    NodeContext& node = EnsureNodeContext(context);
+    if (!node.fee_estimator) {
+        throw JSONRPCError(RPC_INTERNAL_ERROR, "Fee estimation disabled");
+    }
+    return *node.fee_estimator;
+}
+
 /* Calculate the difficulty for a given block index.
  */
 double GetDifficulty(const CBlockIndex* blockindex)