Consistently use atomic operations for all access to DBusConnection refcount

Consistently use atomic operations for all access to DBusConnection refcount

Trying to mix atomic operations with locked non-atomic operations is
broken: the atomic ops aren't necessarily atomic with respect to the
locked non-atomic ops, and the non-atomic ops aren't protected by the
lock because the atomic ops can change the refcount behind their back.

In theory we could use the connection lock if atomic ops aren't supported
(making a per-connection lock cheaper than the global lock used to
implement atomic ops) *and* our mutexes are recursive (making it safe
against deadlocks)... but life's too short.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=38005
Tested-by: Will Manley <freedesktop williammanley net>
Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>