Optimize CurrentInstance.set(UI) and .set(VaadinSession). (#14220)

This change reduces the amount of intermediate steps taken to initialize
the thread locals for a given ui or vaadin session.
It mainly takes advantage of reusing the old values from the map in set()
instead of creating new ones to put into the "old" map which is returned.

Change-Id: I4dbaff973f46e73f8f3a0c285b14c97603f2d5a4