osd: move version setting chicanery into MOSDOpReply.

It doesn't need to be in OSD/ReplicatedPG and we should
factor it out somewhere.

Signed-off-by: Greg Farnum <greg@inktank.com>
update docs on new version API

Signed-off-by: Greg Farnum <greg@inktank.com>
fixup "osd: add last_user_version to pg_info_t"

Set the last_user_version in generate_test_instances and rename the
output

Signed-off-by: Greg Farnum <greg@inktank.com>