wayland: Desynchronize transactions of the same client

At the moment transactions from the same client are applied in the
commit order even if the trasactions affect unrelated surfaces.

This patch desynchronizes transactions affecting unrelated surfaces.

With this, if a client updates two surfaces (as an example, Firefox with
two windows) and one of its surfaces takes longer to render, the other
surface is not going to be slowed down.

Another nice thing is that it removes client from Transaction, which
might be potentially useful to the Workspace for coordinated resize or
something.