server: No longer accept "sideband" and "aes_ctr128" features.

Both features are used unconditionally since commit d44413588dd7
(v0.6.3-27) from three years ago when the client stopped to request
the feature. We don't need to support clients older than that any more,
so fail the request if these features are still requested.

Clarify the comment about the sha256 feature while at it.