LU-1943 osd: fix lost functionality in OFD and OSD

LU-1943 osd: fix lost functionality in OFD and OSD

- restore ofd_fmd_expire() in ofd_ping()
- set OS_STATE_READONLY in osd_statfs()
- fix readcache initialization and add lprocfs entries for it
- fix test_19 to work with OSD if OFD is used

Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Change-Id: I357b28d8d5de1e48c55b12faa4d31ac75dbd5f66
Reviewed-on: http://review.whamcloud.com/3991
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>