block/block-copy: rename in-flight requests to tasks

We are going to use aio-task-pool API and extend in-flight request
structure to be a successor of AioTask, so rename things appropriately.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200429130847.28124-2-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>