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>