messages/: include shard information in various pg messages

messages/: include shard information in various pg messages

We can no longer use the messenger source information to determine
the origin of the message since an osd might have more than one
shard of a particular pg.  Thus, we need to include a pg_shard_t
from field to indicate origin.  Similarly, pg_t is no longer
sufficient to specify the destination pg, we instead use spg_t.
In the event that we get a message from an old peer, we default
from to pg_shard_t(get_source().num(), ghobject_t::no_shard())
and spg_t to spg_t(pgid, ghobject_t::no_shard()).  This suffices
because non-NO_SHARD shards can only appear once ec pools have
been enabled -- and doing that bans unenlightened osds.

Signed-off-by: Samuel Just <sam.just@inktank.com>