decRefcount

Decrement the availability of index (a peer that had it disconnected, or sent dont-have). Mirrors piece_picker::dec_refcount(piece_index_t, ...), including the "break one seed" fix-up: if the per-piece count is already 0 but seeds exist, one seed is converted into explicit per-piece counts first so the counter never goes negative.

Decrement availability for every piece in bitmask (a peer with this bitfield disconnected). Mirrors the bitfield dec_refcount, with the same break-one-seed fix-up per piece.