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We have several Kinesis streams set up with event notifications that trigger lambdas.  When the LAMBDA_EXECUTOR is set to 'docker' and we send a large number or records to Kinesis, multiple Lambdas will execute, but currently they are failing to create with error:

```
Exception: Lambda process returned error status code: 1. Output:
time="2019-09-18T17:28:11Z" level=error msg="error getting events from daemon: net/http: request canceled" 
Error response from daemon: driver failed programming external connectivity on endpoint busy_sanderson (a78e45a41d0ce5b0d8bab6ff54344fc34fe2b4969f55bf568e753121abc8759e): Bind for 0.0.0.0:38763 failed: port is already allocated
```

I believe this is due to the new debug_java_port introduced with this PR https://github.com/localstack/localstack/pull/1523

With this change, you are setting the port in the constructor by using a method known to have a race condition (https://github.com/localstack/localstack/blob/master/localstack/utils/common.py#L354)
(https://gist.github.com/gabrielfalcao/20e567e188f588b65ba2)

This should be changed to ensure unique ports are being issued across all lambdas


data=json.dumps({'FunctionName': 'test-lambda-function', 'EventSourceArn': 'fake-arn'}))
        listResponse = self.client.get('{0}/event-source-mappings/'.format(lambda_api.PATH_ROOT))
        listResult = json.loads(listResponse.get_data())
        eventSourceMappings = listResult.get('EventSourceMappings')
        self.assertEqual(1, len(eventSourceMappings))
        self.assertEqual('Enabled', eventSourceMappings[0]['State'])
    def test_create_disabled_event_source_mapping(self):
        createResponse = self.client.post('{0}/event-source-mappings/'.format(lambda_api.PATH_ROOT),
                            data=json.dumps({'FunctionName': 'test-lambda-function',
                                             'EventSourceArn': 'fake-arn',
                                             'Enabled': 'false'}))
        createResult = json.loads(createResponse.get_data())
        self.assertEqual('Disabled', createResult['State'])
        getResponse = self.client.get('{0}/event-source-mappings/{1}'.format(lambda_api.PATH_ROOT,
                        createResult.get('UUID')))
        getResult = json.loads(getResponse.get_data())
        self.assertEqual('Disabled', getResult['State'])
    def test_update_event_source_mapping(self):
        createResponse = self.client.post('{0}/event-source-mappings/'.format(lambda_api.PATH_ROOT),
                            data=json.dumps({'FunctionName': 'test-lambda-function',
                                             'EventSourceArn': 'fake-arn',
                                             'Enabled': 'true'}))
        createResult = json.loads(createResponse.get_data())
        putResponse = self.client.put('{0}/event-source-mappings/{1}'.format(lambda_api.PATH_ROOT,
                        createResult.get('UUID')), data=json.dumps({'Enabled': 'false'}))
        putResult = json.loads(putResponse.get_data())
        self.assertEqual('Disabled', putResult['State'])
        getResponse = self.client.get('{0}/event-source-mappings/{1}'.format(lambda_api.PATH_ROOT,
                        createResult.get('UUID')))
        getResult = json.loads(getResponse.get_data())