PGLog: initialize complete_to

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: fix logging in trim

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: add rollback_info_trimme_to_riter to IndexedLog and use in PG

Signed-off-by: Samuel Just <sam.just@inktank.com>