Add minPriceUSD in L2DB, check maxTxs atomically

- Add config parameter `Coordinator.L2DB.MinPriceUSD` which allows rejecting
  txs to the pool that have a fee lower than the minimum.
- In pool tx insertion, checking the number of pending txs atomically with the
  insertion to avoid data races leading to more than MaxTxs pending txs in the
  pool.

node/node.go

@@ -205,6 +205,7 @@ func NewNode(mode Mode, cfg *config.Node) (*Node, error) {
 			db,
 			cfg.Coordinator.L2DB.SafetyPeriod,
 			cfg.Coordinator.L2DB.MaxTxs,
+			cfg.Coordinator.L2DB.MinFeeUSD,
 			cfg.Coordinator.L2DB.TTL.Duration,
 			apiConnCon,
 		)