osd: EC Optimizations: Backfill changes for partial writes

osd: EC Optimizations: Backfill changes for partial writes

Optimized EC pools support partial writes that do not update every shard.
Consequently shards that are not updated can have out of date version
numbers. The primary shard object_info_t is always updated and tracks the
expected version of each shards. To avoid unnecessary backfill work changes
are required to use the extra data in the object_info_t when comparing version
numbers to work out whether a shard is missing updates or just didn't
participate in recent partial writes.

See comments in src/osd/recovery_types.h

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