osd: add _fastinfo PG attr for common pg_info_t updates

osd: add _fastinfo PG attr for common pg_info_t updates

For most IO operations we only update a handful of fields
in the pg_info_t structure.  However, the full struct,
when encoded, is on the order of 800 bytes.

This adds a new attribute, _fastinfo, which contains only
the most commonly updated fields.  When present, the
fastinfo fields should be overlayed on top of the full
info struct contained in the existing info attr.  If
a field outside of the "fast" set is updated, we clear
the fastinfo attribute and update the full info attr.

Signed-off-by: Sage Weil <sage@redhat.com>