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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


forward_headers = modified_request.headers
                    break
                elif listener_result is not True:
                    
                    code = listener_result if isinstance(listener_result, int) else 503
                    self.send_response(code)
                    self.send_header('Content-Length', '0')
                    self.end_headers()
                    return
            
            if response is None:
                forward_headers['Connection'] = forward_headers.get('Connection') or 'close'
                data_to_send = self.data_bytes
                request_url = proxy_url
                if modified_request:
                    if modified_request.url:
                        request_url = '%s%s' % (forward_url, modified_request.url)
                    data_to_send = modified_request.data
                response = self.method(request_url, data=data_to_send,
                    headers=forward_headers, stream=True)
                
                if not response._content_consumed and response.raw:
                    response._content = response.raw.read()
            
            if self.proxy.update_listener:
                kwargs = {
                    'method': method,
                    'path': path,
                    'data': data,
                    'headers': forward_headers,