net: zperf: Fix session leak

In case zperf session was aborted by the user (by for instance stopping
it from shell), or practically in case of any other
communication-related error, the zperf session could end up in a state
other than NULL or COMPLETED, with no way to recover. This made the
session no longer usable and eventually could lead to zperf being not
able to start a new session anymore.

Fix this by introducing zperf_session_reset() function, which resets the
session state back to defaults. The function is called when the zperf
receiver service is stopped.

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