fix: Improve supraseal batch ref tracking

[magik6k]

This rewrites the local batch ref tracker to:

• Track individual sectors
• Sync state with the DB

In the local tracker there are now 3 states for "slots":

• No sector refs - batch can take more work
• work=true - currently ongoing batch seal task
• Sector refs, work=false - waiting for C1 / Finalize - refs removed from the DB will eventually be removed from the local tracker

[LexLuthr]

Shouldn't s.lk.Lock() be s.cond.L.Lock()?

[magik6k]

That's the same lock, the cond is created with sm.cond = sync.NewCond(&sm.lk)