The MqttAsyncClient set the number of executorService cores-thread to less than three lead to java.io.EOFException

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- [ ] Bug exists Release Version 1.2.5 ( Master Branch)

If the MqttAsyncClient set the number of executorService cores-thread to less than three
When call it's connect method , it will execute CommsCallback start method ,then this leads to a problem,Just like I drew it in the picture

Start by tracing MqttAsyncClient.connect into CommsCallback.start, focusing on behavior when executorService has fewer than three cores. Reproduce the reported java.io.EOFException on release 1.2.5 or the master branch; done means the connection no longer fails under that configuration and the relevant behavior is covered by verification.

Written by the indexing model from the issue text.