kobo/xmlrpc.py: hold CONNECTION_LOCK while calling make_connection()
kobo/xmlrpc.py: hold CONNECTION_LOCK while calling make_connection()

The same locking code that was used in the CookieTransport class should
be used in the SafeCookieTransport class.  I have no self-contained
reproducer for this bug: Covscan workers occasionally fail to upload
task results to hub over XML-RPC, only if https:// HUB_URL is used
(whereas they work reliably if http:// HUB_URL is used).

release-engineering/kobo,release-engineering/kobo,release-engineering/kobo,release-engineering/kobo