pool: `payout_mode` in `Downstream` should be handled per-channel and not as a single value

[GitGab19]

Every downstream can have multiple channels opened with the Pool, and theoretically these channels could have different payout modes (e.g one channel fully donates to SRI, and another channel fully rewards the miner).

Currently, in Downstream, we have this:

/// Payout mode derived from user_identity (None until channel is opened)
    pub payout_mode: SharedLock<Option<PayoutMode>>,

Instead, that should be a hashmap/dashmap/sharedmap with per-channel values.