IotTaskPool_GetSystemTaskPool has been removed from the cut down version
of the task pool. define IOT_SYSTEM_TASKPOOL to NULL so that the MQTT library
builds successfully.
Use the new #defines in the iot_config.h.
The DNS cache entry size was not big enough to fit the DNS name and
as a result the DNS reply parsing code returned error. Increased the
size of the entry to ensure that the DNS name can fit in.
The example now creates a recyclable job, schedules it and returns it
back to the task pool when it is done. It then again creates a
recyclable job and ensures that the task pool the same job present
it its cache.
