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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.function_invoke_times = {}
        self.debug_java_port = get_free_tcp_port()
    def execute(self, func_arn, func_details, event, context=None, version=None, asynchronous=False):
        def do_execute(*args):
            
            invocation_time = int(time.time() * 1000)
            
            try:
                result, log_output = self._execute(func_arn, func_details, event, context, version)
            finally:
                self.function_invoke_times[func_arn] = invocation_time
            
            self._store_logs(func_details, log_output, invocation_time)
            
            return result, log_output
        
        if asynchronous:
            LOG.debug('Lambda executed in Event (asynchronous) mode, no response from this '
                      'function will be returned to caller')
            FuncThread(do_execute).start()
            return None, 'Lambda executed asynchronously.'
        return do_execute()
    def _execute(self, func_arn, func_details, event, context=None, version=None):