samples: net: echo_client: Fix bug in workqueue processing

The `start_udp_and_tcp` funcion is a blocking function, therefore it
should not be called from the system workqueue, as it would stall it.
Because it was called in such a way, the retry mechanism, which
also relied on the system workqueue did not work properly.

This commit fixes the issue, by keeping the main application processing
in the main thread, and using a semaphore to synchronize with the
connection manager.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>