FileStore::stat: valgrind: don't read *st on error

Signed-off-by: Samuel Just <sam.just@inktank.com>
OpRequest: use OpRequestRef for OpHistory

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: clear OpHistory on shutdown

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