osd: Relax assertion that all recoveries require a read.

osd: Relax assertion that all recoveries require a read.

If multiple object are being read as part of the same recovery (this happens
when recovering some snapshots) and a read fails, then some reads from other
shards will be necessary.  However, some objects may not need to read. In this
case it is only important that at least one read message is sent, rather than
one read message per object is sent.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>
(cherry picked from commit 9f9ea6ddd38ebf6ae7159855267e61858bb2b7fc)