After `reconnect()`, MQTTClient.socket is blocking by default, and
check_msg() can block. This commit aims to fix that behaviour by
reimplementing `check_msg()` for umqtt.robust and setting the socket to
non-blocking.
Fixes issue #192.
Uses the new require()/package()/module() functions from manifestfile.py.
Add manifest.py for iperf3 and pyjwt.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>