HSEARCH-1755 Making sure that DefaultStringBridge has a public no-arg co...

...nstructor

Also allowing the use of @Factory within a FieldBridge. This opens the possibltiy to re-use singleton instances,
even if @FieldBridge is used to specify bridge class.

Renaming some of the methods in BridgeFactory.
> Could you invoke the factory method only after having validated the number of annotated methods?

What's the difference? I guess I can. FYI, this is how it is done somewhere else in the code as well. 

> Bonus points if you add a meaningful error message for A) the method not being accessible B) not having a zero-parameters signature C) returning a wrong type D) returning null E) wrap any runtime exception it might trigger on invocation

Good thinking actually. Maybe I can even extract the whole thing into a helper of sort and refactor the other places which use `@Factory` to use the helper as well. This way, error handling will be consistent and maybe it gets easier to re-use in case we want to use `@Factory` somewhere else.