Reuse peer data with same task id

[jim3ma]

Feature request:

Currently, when downloads peers data with same task id, dfget will write multiple times. We need an option to reuse the underlying data which is already downloaded.

Use case:

two peer with same id, peer1 downloaded first, then peer2 starts.

currently:

• peer1/task-000 -> /data/task-000/peer1
• peer2/task-000 -> /data/task-000/peer2

reuse peer data:

• peer1/task-000 -> /data/task-000/peer1
• peer2/task-000 -> /data/task-000/peer1

when peer1 is ok, peer2 will reuse the peer1 data directly.

UI Example: