fees: make the class FeeFilterRounder thread-safe

So that its methods can be called concurrently by different threads on
the same object. Currently it has just one method (`round()`).

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

src/policy/fees.cpp

@@ -1010,8 +1010,10 @@ FeeFilterRounder::FeeFilterRounder(const CFeeRate& minIncrementalFee)
 CAmount FeeFilterRounder::round(CAmount currentMinFee)
 {
     std::set<double>::iterator it = feeset.lower_bound(currentMinFee);
-    if ((it != feeset.begin() && insecure_rand.rand32() % 3 != 0) || it == feeset.end()) {
+    if (it == feeset.end() ||
-        it--;
+        (it != feeset.begin() &&
+         WITH_LOCK(m_insecure_rand_mutex, return insecure_rand.rand32()) % 3 != 0)) {
+        --it;
     }
     return static_cast<CAmount>(*it);
 }

src/policy/fees.h

@@ -299,12 +299,13 @@ public:
     /** Create new FeeFilterRounder */
     explicit FeeFilterRounder(const CFeeRate& minIncrementalFee);
 
-    /** Quantize a minimum fee for privacy purpose before broadcast. Not thread-safe due to use of FastRandomContext */
+    /** Quantize a minimum fee for privacy purpose before broadcast. */
     CAmount round(CAmount currentMinFee);
 
 private:
     std::set<double> feeset;
-    FastRandomContext insecure_rand;
+    Mutex m_insecure_rand_mutex;
+    FastRandomContext insecure_rand GUARDED_BY(m_insecure_rand_mutex);
 };
 
 #endif // BITCOIN_POLICY_FEES_H