Observable.subscribe().unsubscribe() and ListenableFuture.cancel() (#397)

Observable.subscribe().unsubscribe() dance to free up the MQ resources server-side.

* remove an unused import

* implement a FlowHandle<Transaction>.finalize method

* Rename finalize() to discard() - remove the collection and run discard individually

* Remove unused imports

* Observable.notUsed helper function

* Tweaks to comments

* FlowHandle implements AutoClosable

* Resolving conflicts and move notUsed to RPC module

* Copy Observable.notUsed in core module.

* delete discard method

client/rpc/src/main/kotlin/net/corda/client/rpc/Utils.kt

@@ -0,0 +1,19 @@
+package net.corda.client.rpc
+
+import rx.Observable
+
+/**
+ * This function should be invoked on any unwanted Observables returned from RPC to release the server resources.
+ *
+ * subscribe({}, {}) was used instead of simply calling subscribe()
+ * because if an {@code onError} emission arrives (eg. due to an non-correct transaction, such as 'Not sufficient funds')
+ * then {@link OnErrorNotImplementedException} is thrown. As we won't handle exceptions from unused Observables,
+ * empty inputs are used to subscribe({}, {}).
+ */
+fun <T> Observable<T>.notUsed() {
+    try {
+        this.subscribe({}, {}).unsubscribe()
+    } catch (e: Exception) {
+        // Swallow any other exceptions as well.
+    }
+}