KafkaConnectorTests heisenbug

Following failure was seen on travis

```
Kafka connector should send and receive a kafka message even after session timeout
org.scalatest.exceptions.TestFailedException: 1 was not equal to 3

at org.scalatest.MatchersHelper$.indicateFailure(MatchersHelper.scala:340)
at org.scalatest.Matchers$AnyShouldWrapper.shouldBe(Matchers.scala:6864)
at services.KafkaConnectorTests$$anonfun$2.apply$mcV$sp(KafkaConnectorTests.scala:136)
at services.KafkaConnectorTests$$anonfun$2.apply(KafkaConnectorTests.scala:120)
at services.KafkaConnectorTests$$anonfun$2.apply(KafkaConnectorTests.scala:120)
```
Test output
```
Send message to topic.
Successfully sent message to topic: KafkaConnectorTestTopic-0@5
Receiving message from topic.
Received 1. Took 6 msec: List(Wed Apr 04 06:21:59 UTC 2018)
Send message to topic.
Successfully sent message to topic: KafkaConnectorTestTopic-0@6
Receiving message from topic.
Received 1. Took 5 msec: List(Wed Apr 04 06:21:59 UTC 2018)
Send message to topic.
Successfully sent message to topic: KafkaConnectorTestTopic-0@7
Receiving message from topic.
Received 2. Took 17 msec: List(Wed Apr 04 06:21:59 UTC 2018, Wed Apr 04 06:22:10 UTC 2018)
Send message to topic.
Successfully sent message to topic: KafkaConnectorTestTopic-0@8
Receiving message from topic.
Received 2. Took 13 msec: List(Wed Apr 04 06:21:59 UTC 2018, Wed Apr 04 06:22:10 UTC 2018)
Send message to topic.
Successfully sent message to topic: KafkaConnectorTestTopic-0@9
Receiving message from topic.
Received 1. Took 4 msec: List(Wed Apr 04 06:22:21 UTC 2018)
```
See in
* https://scans.gradle.com/s/ith6wms6wtnua/tests/failed ([travis](https://travis-ci.org/apache/incubator-openwhisk/builds/361978262) (4-April-2018)
* https://scans.gradle.com/s/p4wqdppt34ilo/tests/failed