osd: EC Optimizations: Partial write changes to add_next_event

osd: EC Optimizations: Partial write changes to add_next_event

add_next_event is used during peering to process log entries
that a shard is missing to build up a list of missing objects.
With EC optimized pools and partial writes not every update
modifies every shard. The log entry contains details of which
shards were modified and this can be used to work out whether
a missing entry needs to be created/updated.

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