The number of threads in DefaultMQProducerImpl/defaultAsyncSenderExecutor can be dynamically configured？

[zhangjidi2016]

FEATURE REQUEST

1. Please describe the feature you are requesting.
   The number of threads in DefaultMQProducerImpl/defaultAsyncSenderExecutor can be dynamically configured？
2. Provide any additional detail on your proposed use case for this feature.
   

[fibbery]

你可以自己定义发送的线程池的，然后你就可以动态设置了