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.

db/migrations/0001.sql

@@ -47,7 +47,7 @@ CREATE TABLE token (
     name VARCHAR(20) NOT NULL,
     symbol VARCHAR(10) NOT NULL,
     decimals INT NOT NULL,
-    usd NUMERIC,
+    usd NUMERIC, -- value of a normalized token (1 token = 10^decimals units)
     usd_update TIMESTAMP WITHOUT TIME ZONE
 );