Remove incremental header compression mode.

When the `HPACKEncoder` was first written it was written to provide incremental header compression. This was enhanced later on in the development of v1 to provide one-shot header compression, which is what `NIOHTTP2` uses today.

The presence of the incremental compression mode with `beginEncoding`/`endEncoding` forces some awkward compromises in the one-shot compression mode, which means that `beginEncoding`/`endEncoding` are forced to do weird extra allocations for no particularly good reason. Of course, this doesn't matter really, because we don't use the functions anyway, but they're part of the public interface to `NIOHPACK` and so cannot be changed right now.

In v2 we should just delete those functions. They're not useful, and cause more complexity than necessary.

Start by locating the HPACKEncoder implementation and its beginEncoding/endEncoding entry points, then inspect how NIOHTTP2 uses one-shot header compression. Confirm the v2 API and compatibility implications before removing the incremental mode; done means the one-shot path remains functional without the incremental-compression compromises.

Written by the indexing model from the issue text.