prserv: Fix exit race issues

We shouldn't immediately remove the pid file when stopping the server, if we do, this
causes a traceback within the server itself which can then hang. Fix this by removing
the stale pid file as the last thing we do.

Also:

* don't printing a new "waiting" line every 0.5 seconds.
* make the loop more granular since the user can 'feel' the 0.5 seconds

[YOCTO #5984]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>