Constant cfxcore::sync_parameters::CATCH_UP_EPOCH_LAG_THRESHOLD

pub const CATCH_UP_EPOCH_LAG_THRESHOLD: u64 = 20; // 20u64

The threshold controlling whether a node is in catch-up mode. A node is in catch-up mode if its local best epoch number is CATCH_UP_EPOCH_LAG_THRESHOLD behind the median of the epoch numbers of peers.