Socket binding implemented properly for IPv6 and UNIX sockets. (#1641)

* Socket binding implemented properly for IPv6 and UNIX sockets.

- app.run("::1") for IPv6
- app.run("unix:/tmp/server.sock") for UNIX sockets
- app.run("localhost") retains old functionality (randomly either IPv4 or IPv6)

Do note that IPv6 and UNIX sockets are not fully supported by other Sanic facilities.
In particular, request.server_name and request.server_port are currently unreliable.

* Fix Windows compatibility by not referring to socket.AF_UNIX unless needed.

* Compatibility fix.

* Fix test of existing unix socket.

* Cleaner unix socket removal.

* Remove unix socket on exit also with workers=1.

* More pedantic UNIX socket implementation.

* Refactor app to take unix= argument instead of unix:-prefixed host. Goin' fast @ unix-socket fixed.

* Linter

* Proxy properties cleanup. Slight changes of semantics. SERVER_NAME now overrides everything.

* Have server fill in connection info instead of request asking the socket.

- Would be a good idea to remove request.transport entirely but I didn't dare to touch it yet.

* Linter 💣🌟✊💀

* Fix typing issues. request.server_name returns empty string if host header is missing.

* Fix tests

* Tests were failing, fix connection info.

* Linter nazi says you need that empty line.

* Rename a to addr, leave client empty for unix sockets.

* Add --unix support when sanic is run as module.

* Remove remove_route, deprecated in 19.6.

* Improved unix socket binding.

* More robust creating and unlinking of sockets. Show proper and not temporary name in conn_info.

* Add comprehensive tests for unix socket mode.

* Hide some imports inside functions to avoid Windows failure.

* Mention unix socket mode in deployment docs.

* Fix merge commit.

* Make test_unix_connection_multiple_workers pickleable for spawn mode multiprocessing.

Co-authored-by: L. Kärkkäinen <tronic@users.noreply.github.com>
Co-authored-by: Adam Hopkins <admhpkns@gmail.com>

examples/delayed_response.py

@@ -0,0 +1,18 @@
+from asyncio import sleep
+
+from sanic import Sanic, response
+
+app = Sanic(__name__, strict_slashes=True)
+
+@app.get("/")
+async def handler(request):
+    return response.redirect("/sleep/3")
+
+@app.get("/sleep/<t:number>")
+async def handler2(request, t=0.3):
+    await sleep(t)
+    return response.text(f"Slept {t:.1f} seconds.\n")
+
+
+if __name__ == '__main__':
+    app.run(host="0.0.0.0", port=8000)