Refactoring related to BFT state persistence/transfer (#829)

* Remove unused type param
* If we drop Runnable we can use conciser syntax
* Sometimes we need the fully-fledged object, so retire separate handle class
* Implement IntelliJ suggestion

test-utils/src/main/kotlin/net/corda/testing/node/MockNode.kt

@@ -243,7 +243,7 @@ class MockNetwork(private val networkSendManuallyPumped: Boolean = false,
         }
 
         fun manuallyCloseDB() {
-            dbCloser?.run()
+            dbCloser?.invoke()
             dbCloser = null
         }