MOVEONLY: DisconnectedBlockTransactions to its own file

This struct is only used in validation + tests and has very little to do
with txmempool.

src/test/validation_chainstatemanager_tests.cpp

@@ -4,6 +4,7 @@
 //
 #include <chainparams.h>
 #include <consensus/validation.h>
+#include <kernel/disconnected_transactions.h>
 #include <node/kernel_notifications.h>
 #include <node/utxo_snapshot.h>
 #include <random.h>