osd: EC Optimizations: Get missing changes for partial logs

osd: EC Optimizations: Get missing changes for partial logs

Changes to the get missing step of peering to handle partial
writes. Having established the authorative log the primary
works out what shards are missing objects. With partial
writes this code needs to differentiate between a shard that
missed an update (and hence has a missing object) versus a
shard that was not updated by a partial write. The divergent
log entries are examined to see if the updates were partial
writes that did not involve the shard.

Signed-off-by: Bill Scales <bill_scales@uk.ibm.com>