net: samples: echo-client: Allow UDP and TCP run at the same time

Run both TCP and UDP to run at the same time. This requires two
extra threads in echo-client sample application.

Change-Id: I450a26d7ab1472150ea8f1309a43897a30bd4d90
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>