make API independent from impl packages

As discussed on the developer mailing list, I would feel better if our API packages in HttpCore would not depend on impl packages.
There used to be two classes in impl that are referenced from the API:

DefaultHttpParams - could be moved to API params package and renamed to BasicHttpParams. The renaming would also prevent the misconception that we might have some kind of "default parameters" that are automatically loaded by HttpCore.

HeaderGroup - this one fould fit nicely in the o.a.h.util package. We already have ByteArrayBuffer and CharArrayBuffer there although neither is in the actual API.

Recently, HTTPCORE-39 introduced a third dependency, on EnglishReasonPhraseCatalog. Unless that dependency is removed, there is no point in breaking the API for these two here.