Change HTTPClient handler factory creation

[sacovo]

The handler factory was supplied in the apply method of a client together with the Payload for the request. If the client already had a session for the new host, port and scheme, no new session was created and the supplied handler factory wasn't used.

The new design makes it clear, that one client uses the same handler factory for all requests it makes. If different handler factories are needed, different clients need to be created.

[ponylang-main]

Hi @sacovo,

The changelog - changed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 106.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

@sacovo thank you. as part of your release notes, please include a "how to upgrade" portion that shows what example old version code would look like and what the new version looks like.