ping: fixing wrong value when there are worker threads

- the echo_reply_node is now notifying the cli process on the main thread/vlib_main
- the timestamp for the icmp reply is now acquired in the echo_reply_node and not in the cli process to avoid an off by 10ms error (see 【vpp-dev】delay is error in ping with multi worker thread)

Change-Id: I21d37002b0376b4f2ccab08d8f04c2f2944b9b39
Signed-off-by: Mohammed Hawari <mhawari@cisco.com>