CCAN: Use it from our mirror

The reason is some people are blocked to access git:// protocol/port
because of firewall setup in their environment, and git.ozlabs.org does
not provide a way to clone over http(s).

We can provide CCAN mirror and we support cloning it over http(s). Also,
it makes sense to not depend on thirdparty sites for hosting
strongly-related components.

.gitmodules

 [submodule "ccan"]
 	path = 3rdparty/ccan
-	url = git://git.ozlabs.org/~ccan/ccan
+
+# original url:
+#	url = git://git.ozlabs.org/~ccan/ccan
+# but people are sometimes blocked to access git:// because of firewalls.
+#
+# Also it is better to not depend on thirdparty hosting sites
+	url = https://lab.nexedi.cn/mirror/ccan.git