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>
length() must equal to chunk length. Or decode will trigger assert.
So we just move the assert from length not match to item not match. I think we don't need to add flag check here.