- 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.
Update usage of `*babyjub.PublicKey` to `babyjub.PublicKeyComp`
- when the key is not defined, internally is used `babyjub.EmptyBJJComp`, which is a `[32]byte` of zeroes of type `babyjub.PublicKeyComp`
- the API continues returning `nil` when the key is not defined
