ssynchronized: act on a class member

The ssynchronized decorator takes the lock to act on in input.
With this change we allow a string to be passed, and if so we assume the
function it protects is a class method, and we act on the member of the
class itself named as the string we got.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>