Register() call in Iotcc is not thread-safe

[pmasrani]

https://github.com/vmware/liota/blob/master/liota/dccs/iotcc.py#L135

In Iotcc, register() call sends a message and awaits the response on a queue. The queue implementation is not thread-safe and if multiple listener threads are awaiting on the register() call, it can lead to inconsistent behavior due to out of order messages