fixup! [tx fees, policy]: create a mempool fee estimator

* Always calculate fee rates for MAX_CONF_TARGET number of blocks, as
  it's not that expensive to do so, and we cache them all.

src/policy/mempool_fees.cpp

@@ -28,25 +28,23 @@ CFeeRate MemPoolPolicyEstimator::EstimateFeeWithMemPool(Chainstate& chainstate,
         err_message = "Mempool did not finish loading, can't get accurate fee rate estimate.";
         return CFeeRate(0);
     }
-    // Try the cache if not forced
     if (!force) {
         cached_fee = cache.get(confTarget);
     }
 
     if (!cached_fee) {
-        // Run block builder and update cache
         std::map<CFeeRate, uint64_t> mempool_fee_stats;
-        size_t target_weight = confTarget * DEFAULT_BLOCK_MAX_WEIGHT;
-        mempool_fee_stats = GetCustomBlockFeeRateHistogram(chainstate, &mempool, target_weight);
+        // Always get stats for MAX_CONF_TARGET blocks because current algo
+        // fast enough while we're here
+        mempool_fee_stats = GetCustomBlockFeeRateHistogram(chainstate, &mempool, DEFAULT_BLOCK_MAX_WEIGHT * MAX_CONF_TARGET);
         if (mempool_fee_stats.empty()) {
             err_message = "No transactions available in the mempool yet.";
             return CFeeRate(0);
         }
-        fee_rates = EstimateBlockFeeRatesWithMempool(mempool_fee_stats, confTarget);
+        fee_rates = EstimateBlockFeeRatesWithMempool(mempool_fee_stats, MAX_CONF_TARGET);
         cache.update(fee_rates);
         block_fee_rate = fee_rates[confTarget];
     } else {
-        // Use the cached value
         block_fee_rate = *cached_fee;
     }
 
@@ -60,7 +58,6 @@ std::map<uint64_t, CFeeRate> MemPoolPolicyEstimator::EstimateBlockFeeRatesWithMe
     const std::map<CFeeRate, uint64_t>& mempool_fee_stats, unsigned int confTarget) const
 {
     std::map<uint64_t, CFeeRate> fee_rates;
-     // Return empty if no stats
     if (mempool_fee_stats.empty()) return fee_rates;
 
     auto start = mempool_fee_stats.begin();