pool: include source pool name into pp ls output

Motivation:
When pool performs a migration then with `pp ls` admins can see the
ongoing transfers. however, the source pools are not shown.

Modification:
Include source pool into output of `pp ls` command.

Result:
Better observability.

before:

```
[dcache-lab008] (poolA@poolA) admin > \s poolB pp ls
124 000014895EDDD7CF44F2AF3055AC89945F34 FSM.Transferring
```

after:

```
[dcache-lab008] (poolB@poolB) admin > \s poolA pp ls
101 0000367F9C7C3E6848DA906D4C210FE17076 FSM.Transferring << poolB
```

Acked-by: Karen Hoyos
Target: master
Require-book: no
Require-notes: yes

modules/dcache/src/main/java/org/dcache/pool/p2p/Companion.java

@@ -265,7 +265,7 @@ public synchronized boolean cancel(Object cause) {
  public String toString() {
  // Unsynchronized access to the fsm state means we may show an old value, but it
  // avoids blocking in toString().
- return _id + " " + _pnfsId + " " + _fsm.getState();
+ return _id + " " + _pnfsId + " " + _fsm.getState() + " << " + _sourcePoolName;
  }
 
  /**