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>
No. get_chunk_size is always 512. But j->second.length normally is object_size/k. which is my previous check-in. But we found that in recovering-op this may not true. 
In fact in decode it checks
1. All sub-read have same size.
2. size is multiple of chunk_size.
But we can't check 1 here because we don't know which size is correct if we get difference size.

I still think we can get the correct data size but my previous is wrong. And even this check is weak it still can filter some errors.