fix(pw): insert to active_sessions, avoid creating duplicated threads

[taoky]

In daemon/src/pw.rs, pipewire_service() with pw_tx and pw_rx:

• session_monitor checks /run/user/pipewire-0 every 60s, and if find one, send SocketEvent::Add(socket) from pw_tx
• session_spawner receives from pw_rx, and creates a thread if it is SocketEvent::Add(socket)

BTreeSet<PathBuf> is defined to "deduplicate" socket names. However, it seems that this BTreeSet is always empty, as no one is inserting anything into it. Thus this daemon will actually create a thread every 60s, probably an unexpected behavior.

[mmstick]

I'll merge your PR into the scheduler branch so that they can be reviewed alongside other PRs and QA in one pass.