tools/net/ynl: improve async notification handling

The notification handling in ynl is currently very simple, using sleep()
to wait a period of time and then handling all the buffered messages in
a single batch.

This patch changes the notification handling so that messages are
processed as they are received. This makes it possible to use ynl as a
library that supplies notifications in a timely manner.

- Change check_ntf() to be a generator that yields 1 notification at a
  time and blocks until a notification is available.
- Use the --sleep parameter to set an alarm and exit when it fires.

This means that the CLI has the same interface, but notifications get
printed as they are received:

./tools/net/ynl/cli.py --spec <SPEC> --subscribe <TOPIC> [ --sleep <SECS> ]

Here is an example python snippet that shows how to use ynl as a library
for receiving notifications:

    ynl = YnlFamily(f"{dir}/rt_route.yaml")
    ynl.ntf_subscribe('rtnlgrp-ipv4-route')

    for event in ynl.check_ntf():
        handle(event)

Signed-off-by: Donald Hunter <[email protected]>
Tested-by: Kory Maincent <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

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