Add fallback resolvers for CurrentInstance (#10974)

Add fallback resolvers for CurrentInstance (#10974)

* Add fallback resolvers for CurrentInstance

This allow applications to inject custom default instances when the
current instances cannot be found by regular means.

For example, when VaadinServlet.getCurrent() would return null, a
fallback resolver could be invoked to properly create the servlet and
return it.

* Make the setting of CurrentInstanceFallbackResolvers protected

* Remove the default constructor. Improve test.

* Made setFallbackResolver public again

* Rename the method to defineFallbackResolver, and make it throw when a
type is used twice

* Make the method thread-safe

* Make the method thread-safe in a Java 6 way

* Thread safety with ConcurrentHashMap API instead of just Map

* Improve test with fake classes.

* Clear the test state after it has been run.