txgraph: Track multiple potential would-be clusters in Trim (improvement)

In the existing Trim function, as soon as the set of accepted transactions
would exceed the max cluster size or count limit, the acceptance loop is
stopped, removing all later transactions. However, it is possible that by
excluding some of those transactions the would-be cluster splits apart into
multiple would-clusters. And those clusters may well permit far more
transactions before their limits are reached.

Take this into account by using a union-find structure inside TrimTxData to
keep track of the count/size of all would-be clusters that would be formed
at any point, and only reject transactions which would cause these resulting
partitions to exceed their limits.

This is not an optimization in terms of CPU usage or memory; it just
improves the quality of the transactions removed by Trim().

src/bench/txgraph.cpp

@@ -112,6 +112,8 @@ void BenchTxGraphTrim(benchmark::Bench& bench)
     });
 
     assert(!graph->IsOversized());
+    // At least 99% of chains must survive.
+    assert(graph->GetTransactionCount() >= (NUM_TOP_CHAINS * NUM_TX_PER_TOP_CHAIN * 99) / 100);
 }
 
 } // namespace