osd: Implement sync reads and sparse reads for EC for direct reads

osd: Implement sync reads and sparse reads for EC for direct reads

Sparse reads for EC are simple to implement, as the code is essentially
identical to that of replica, with some address translation.

When doing a direct read in EC, only a single OSD is involved and
that OSD, by definition is the only OSD involved. As such we can
do the more performant sync read, rather than async read.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>