Fixed some problem I discovered while working on another story (#3901)

* - Fixed some problems with error handling for Observables.
- Eliminated incorrect double `stop()` call for RpcBroker.
- Added `Schedulers.shutdown()` call in `stop()` implementation for Node and Driver, to avoid stuck processes when observable pipelines go wrong.

* Fixed a missing import.

* Removed `Schedulers.shutdown()` for now.

* Fixed an issue with `pendingFlowsCount()` function.

node/src/main/kotlin/net/corda/node/internal/Node.kt

@@ -272,7 +272,6 @@ open class Node(configuration: NodeConfiguration,
                         ArtemisRpcBroker.withoutSsl(configuration.p2pSslOptions, this.address, adminAddress, securityManager, MAX_RPC_MESSAGE_SIZE, jmxMonitoringHttpPort != null, rpcBrokerDirectory, shouldStartLocalShell())
                     }
                 }
-                rpcBroker!!.closeOnStop()
                 rpcBroker!!.addresses
             }
         }

node/src/main/kotlin/net/corda/node/services/messaging/P2PMessagingClient.kt

@@ -310,12 +310,9 @@ class P2PMessagingClient(val config: NodeConfiguration,
                     return
                 }
                 eventsSubscription = p2pConsumer!!.messages
-                        .doOnError { error -> throw error }
-                        .doOnNext { message -> deliver(message) }
                         // this `run()` method is semantically meant to block until the message consumption runs, hence the latch here
                         .doOnCompleted(latch::countDown)
-                        .doOnError { error -> throw error }
-                        .subscribe()
+                        .subscribe({ message -> deliver(message) }, { error -> throw error })
                 p2pConsumer!!
             }
             consumer.start()