Amortize cost of searching for continueKey

VersionedMap.cpp

@@ -801,7 +801,19 @@ struct __attribute__((__visibility__("hidden"))) VersionedMap::Impl {
 
   void printInOrderHelper(int64_t version, uint32_t node, int depth);
 
+  int accumulatedFuel = 0;
+
   void scanAndRemoveOldEntries(int fuel) {
+    accumulatedFuel += fuel;
+#ifdef NDEBUG
+    // This is here for performance reasons, since we want to amortize the cost
+    // of searching for continueKey. In tests, we want to exercise the rest of
+    // the code often.
+    if (accumulatedFuel < 500) {
+      return;
+    }
+#endif
+
     // Get a finger to the first entry > continueKey, or the last entry if
     // continueKey is the empty string
     if (latestRoot == 0) {
@@ -838,7 +850,7 @@ struct __attribute__((__visibility__("hidden"))) VersionedMap::Impl {
 
     // Phew. Ok now we have a finger to the next entry to consider removing, and
     // the range version terminated at this entry.
-    for (; fuel > 0; --fuel) {
+    for (; accumulatedFuel > 0; --accumulatedFuel) {
       const auto &n = mm.base[finger.backNode()];
       if (rangeVersion < 0 && std::max(n.entry->pointVersion,
                                        n.entry->rangeVersion) < oldestVersion) {