OSD: dirty big_info in build_past_intervals_parallel

Otherwise, the newly generated intervals won't get written!

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: use helpers to generate info and epoch keys

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: minor style change for append_log and read_info

They exceeded 80 characters on a line!

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: place biginfo on the infos object next to the info and epoch keys

This is simpler and possibly more efficient.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: init infos_object in OSD::init()

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG,OSD: convert write_info users to write_if_dirty

Also, explicitely dirty info and biginfo where necessary.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge branch 'wip_4113'

Reviewed-by: Sage Weil <sage@inktank.com>