bitbake: cooker/siggen: Support exit calls and use for hashserv client

We have shutdown functions within the async client code but the siggen
doesn't currently call them. We notice on python 3.10 (such as on fedora35)
that at exit, there is a stray asyncio process left behind. Usually this
doesn't cause problems but it could potentially be a cause of a hang.

For general cleanliness and completness, add in hooks to call the exit handler.

(Bitbake rev: 9ee3fb95330003878fbd64b3ce8897aad96fcd0f)

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

bitbake/lib/bb/cooker.py

@@ -1717,6 +1717,7 @@ class BBCooker:
     def post_serve(self):
         self.shutdown(force=True)
         prserv.serv.auto_shutdown()
+        bb.parse.siggen.exit()
         if self.hashserv:
             self.hashserv.process.terminate()
             self.hashserv.process.join()