Add new mempool benchmarks for a complex pool

src/Makefile.bench.include

@@ -30,6 +30,7 @@ bench_bench_bitcoin_SOURCES = \
   bench/gcs_filter.cpp \
   bench/merkle_root.cpp \
   bench/mempool_eviction.cpp \
+  bench/mempool_stress.cpp \
   bench/rpc_blockchain.cpp \
   bench/rpc_mempool.cpp \
   bench/util_time.cpp \