erasure code: backend feature to read from all OSDs and silently discard EIO if possible

Bug: http://tracker.ceph.com/issues/9943

Brief: Add osd setting osd_pool_erasure_code_subread_all. Default is false. If set it to true. Will send sub-read op to all OSD and choose the first success read result.

Signed-off-by: Wei Luo <luowei@yahoo-inc.com>
I think you means "assert(0)"?
I think it's hard to read for a condition to assert here. If read failed. Then the errors set and it still assert at finish. 
I will set condition on finish check but remove this assert.