Allow to query single pool transactions

[arnaubennassar]

Right now there is only one method for getting information on transactions on the pool: transaction.GetTransactionsInPool. According to the function description: GetTransactionsInPool connects to a hermez node and pull all transactions in the pool, which is not true because it query the first 1000 txs not all of them. But most importantly, most of the time users will want to query just one tx base on the ID.

Pleae add the method: GetTransactionInPool that returns a single transaction based on tx ID