Add back pressure buffering to stream processor.

This seemed to have been fixed for Observable.from in
ReactiveX/RxJava#3893, as reported in
https://github.com/ReactiveX/RxJava/issues/3892but it's happening
here. Adding onBackpressureBuffer after switchIfEmpty looks to
help the situation as per this patch

For #100.

nakadi-java-client/src/main/java/nakadi/LoggingStreamObserver.java

@@ -13,6 +13,8 @@ public class LoggingStreamObserver extends StreamObserverBackPressure<String> {
 
   private static final Logger logger = LoggerFactory.getLogger(NakadiClient.class.getSimpleName());
 
+  static int eventCount = 0;
+
   @Override public void onStart() {
     logger.info("LoggingStreamObserver.onStart");
   }
@@ -49,7 +51,7 @@ public class LoggingStreamObserver extends StreamObserverBackPressure<String> {
               "LoggingStreamObserver events processing count {} =====================================",
               events.size());
           for (String event : events) {
-            logger.info("LoggingStreamObserver received event: {}", event);
+            logger.info("LoggingStreamObserver received count {} event: {}", ++eventCount, event);
           }
           offsetObserver.onNext(record.streamCursorContext());
           logger.info(

nakadi-java-client/src/main/java/nakadi/StreamProcessor.java

@@ -301,7 +301,8 @@ private <T> Func1<? super Response, Observable<StreamBatchRecord<T>>> observable
       final BufferedReader br = new BufferedReader(response.responseBody().asReader());
       return Observable.from(br.lines()::iterator)
           .map(r -> lineToStreamBatchRecord(r, typeLiteral, response, sc))
-          .switchIfEmpty(forEmpty);
+          .switchIfEmpty(forEmpty)
+          .onBackpressureBuffer();
     };
   }