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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


self.wfile.write(to_bytes(response.content))
        except Exception as e:
            trace = str(traceback.format_exc())
            conn_errors = ('ConnectionRefusedError', 'NewConnectionError',
                           'Connection aborted', 'Unexpected EOF', 'Connection reset by peer')
            conn_error = any(e in trace for e in conn_errors)
            error_msg = 'Error forwarding request: %s %s' % (e, trace)
            if 'Broken pipe' in trace:
                LOG.warn('Connection prematurely closed by client (broken pipe).')
            elif not self.proxy.quiet or not conn_error:
                LOG.error(error_msg)
                if os.environ.get(ENV_INTERNAL_TEST_RUN):
                    
                    
                    
                    print('ERROR: %s' % error_msg)
            self.send_response(502)  
            self.end_headers()
            
            self.close_connection = 1
        finally:
            try:
                self.wfile.flush()
            except Exception as e:
                LOG.warning('Unable to flush write file: %s' % e)
    def _listeners(self):
        return self.DEFAULT_LISTENERS + [self.proxy.update_listener]
    def log_message(self, format, *args):
        return
class GenericProxy(FuncThread):