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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


""" This interceptor method is called by the proxy when returning a response
            (*after* having forwarded the request and received a response from the backend
            service). It receives details of the incoming request as well as the response
            from the backend service, and returns either of the following results:
            * An instance of requests.models.Response to return to the client instead of the
              actual response returned from the backend service.
            * Any other value, in which case the response from the backend service is
              returned to the client.
        """
        return None
    def get_forward_url(self, method, path, data, headers):
        """ Return a custom URL to forward the given request to. If a falsy value is returned,
            then the default URL will be used.
        """
        return None
class GenericProxyHandler(BaseHTTPRequestHandler):
    
    DEFAULT_LISTENERS = []
    def __init__(self, request, client_address, server):