make DisconnectedBlockTransactions responsible for its own memory management

Co-authored-by: Cory Fields <cory-nospam-@coryfields.com>

src/test/validation_chainstatemanager_tests.cpp

@@ -537,7 +537,7 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_snapshot_init, SnapshotTestSetup)
     // it will initialize instead of attempting to complete validation.
     //
     // Note that this is not a realistic use of DisconnectTip().
-    DisconnectedBlockTransactions unused_pool;
+    DisconnectedBlockTransactions unused_pool{MAX_DISCONNECTED_TX_POOL_SIZE * 1000};
     BlockValidationState unused_state;
     {
         LOCK2(::cs_main, bg_chainstate.MempoolMutex());