Bump to version 1.2.1.0 and add missing /Since/-annotations

The /Since/-annotations and version bump are
due to newly exported symbols (added via 7789d1cc)

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Convert changelog to markdown

(and update changelog while at it)

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>