maybeDecreaseCapacity policy was too strong

We can allow larger capacities and still pay for the key bytes

ConflictSet.cpp

@@ -292,9 +292,10 @@ static_assert(kNode3Surplus >= kMinNodeSurplus);
 static_assert(kBytesPerKey - sizeof(Node0) >= kMinNodeSurplus);
 
 // setOldestVersion will additionally try to maintain this property:
-//     `max(children, 1) * length >= capacity`
+//     `(children + entryPresent) * length >= capacity`
 //
-// Which should give us the budget to pay for the key bytes
+// Which should give us the budget to pay for the key bytes. (children +
+// entryPresent) is a lower bound on how many keys these bytes are a prefix of
 
 template <class T, int64_t kMemoryBound = (1 << 20)>
 struct BoundedFreeListAllocator {
@@ -910,7 +911,7 @@ void makeCapacityAtLeast(Node *&self, int capacity, NodeAllocators *allocators,
 void maybeDecreaseCapacity(Node *&self, NodeAllocators *allocators,
                            ConflictSet::Impl *impl) {
   const int maxCapacity =
-      std::max<int>(self->numChildren, 1) * self->partialKeyLen;
+      (self->numChildren + int(self->entryPresent)) * self->partialKeyLen;
   if (self->partialKeyCapacity <= maxCapacity) {
     return;
   }