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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.method = None
        self.headers['Connection'] = self.headers.get('Connection') or 'keep-alive'
        self.forward('CONNECT')
    def read_content(self):
        content_length = self.headers.get('Content-Length')
        if content_length:
            self.data_bytes = self.rfile.read(int(content_length))
            return
        self.data_bytes = None
        if self.method in (requests.post, requests.put):
            LOG.warning('Expected Content-Length header not found in POST/PUT request')
            
            
            socket_timeout_secs = 0.5
            self.request.settimeout(socket_timeout_secs)
            block_length = 1
            while True:
                try:
                    
                    tmp = self.rfile.read(block_length)
                    if self.data_bytes is None:
                        self.data_bytes = tmp
                    else:
                        self.data_bytes += tmp
                except socket.timeout:
                    break
    def build_x_forwarded_for(self, headers):