crimson/osd: drop a foreign-copy to shard-0 for every pg operation

crimson/osd: drop a foreign-copy to shard-0 for every pg operation

By using ConnectionRef before pg submission, and after that, change to
use ConnectionXcoreRef.

The intent is to drop the foreign copy of the connection to shard 0 at
pg submission time. This should remove two pairs of crosscore
communications in shard 0 for each I/O, one for connection-ref foreign
copy, another for connection-ref destruction.

Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>