ReplicatedPG: don't bless C_OSD_SendMessageOnConn

C_OSD_SendMessageOnConn doesn't need to lock the pg.
Canceling it resulted in a leaked message.

Fixes: 6443
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #669 from ceph/wip-6443

ReplicatedPG: don't bless C_OSD_SendMessageOnConn

Reviewed-by: Sage Weil <sage@inktank.com>