net: zperf: Add concurrency when doing upload tests

If user has enabled CONFIG_ZPERF_SESSION_PER_THREAD, then if
user gives -a (async) option to upload command, then multiple
uploads can be run simultaneously. Each upload will be run
in a dedicated work queue. The work queue thread priority can
be set by -t option.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>