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.

cli/node/cfg.buidler.toml

@@ -67,6 +67,7 @@ BJJ = "0x1b176232f78ba0d388ecc5f4896eca2d3b3d4f272092469f559247297f5c0c13"
 [Coordinator.L2DB]
 SafetyPeriod = 10
 MaxTxs       = 512
+MinFeeUSD    = 0.0
 TTL          = "24h"
 PurgeBatchDelay = 10
 InvalidateBatchDelay = 20

cli/node/main.go

@@ -173,6 +173,7 @@ func cmdDiscard(c *cli.Context) error {
 		db,
 		cfg.Coordinator.L2DB.SafetyPeriod,
 		cfg.Coordinator.L2DB.MaxTxs,
+		cfg.Coordinator.L2DB.MinFeeUSD,
 		cfg.Coordinator.L2DB.TTL.Duration,
 		nil,
 	)