Remove old Buffer branching APIs

[maxbrunsfeld]

Buffer::branch was one API of many standing in the way of adopting a new CRDT as the basis for our buffer. This PR removes it, and adjusts its one legitimate user, Buffer::preview_edits to use a different, simpler API, BufferSnapshot::edit, that lets us apply edits a buffer snapshot that will never be merged into the main buffer.

Release Notes:

• N/A

[maxbrunsfeld]

@as-cii I changed the method to Buffer::snapshot_with_edits to avoid the problematic case of making repeated edits to a snapshot. I stayed with the distinct replica id for now, to avoid having to tick the main buffer's lamport every time we preview an edit prediction.