<!-- Love localstack? Please consider supporting our collective:
👉  https://opencollective.com/localstack/donate -->
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


CORS_ALLOWED_HEADERS = ('authorization', 'content-type', 'content-md5', 'cache-control',
    'x-amz-content-sha256', 'x-amz-date', 'x-amz-security-token', 'x-amz-user-agent',
    'x-amz-acl', 'x-amz-version-id')
if EXTRA_CORS_ALLOWED_HEADERS:
    CORS_ALLOWED_HEADERS += tuple(EXTRA_CORS_ALLOWED_HEADERS.split(','))
CORS_ALLOWED_METHODS = ('HEAD', 'GET', 'PUT', 'POST', 'DELETE', 'OPTIONS', 'PATCH')
CORS_EXPOSE_HEADERS = ('x-amz-version-id',)
if EXTRA_CORS_EXPOSE_HEADERS:
    CORS_EXPOSE_HEADERS += tuple(EXTRA_CORS_EXPOSE_HEADERS.split(','))
LOG = logging.getLogger(__name__)
class ThreadedHTTPServer(ThreadingMixIn, HTTPServer):
    """Handle each request in a separate thread."""
    daemon_threads = True
class ProxyListener(object):
    def forward_request(self, method, path, data, headers):
        """ This interceptor method is called by the proxy when receiving a new request
            (*before* forwarding the request to the backend service). It receives details
            of the incoming request, and returns either of the following results:
            * True if the request should be forwarded to the backend service as-is (default).
            * An integer (e.g., 200) status code to return directly to the client without
              calling the backend service.
            * An instance of requests.models.Response to return directly to the client without
              calling the backend service.
            * An instance of requests.models.Request which represents a new/modified request
              that will be forwarded to the backend service.
            * Any other value, in which case a 503 Bad Gateway is returned to the client
              without calling the backend service.
        """
        return True
    def return_response(self, method, path, data, headers, response):