osd: Make casting to/from shard_id_t require explicit casts.

osd: Make casting to/from shard_id_t require explicit casts.

This also required putting in many explicit casts.  I avoided large amount of refactoring to the new shard for now,
although I expect some may follow.  The main aim here is that the new EC code is strict about using the correct shard_id_t
and that confusion between shards, raw shards, OSDs, etc.. flag up as errors.

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