Get a transaction in the mempool by its Transaction Identifier. This is a separate request than fetching a block transaction (/block/transaction) because some blockchain nodes need to know that a transaction query is for something in the mempool instead of a transaction in a block. Transactions may not be fully parsable until they are in a block (ex: may not be possible to determine the fee to pay before a transaction is executed). On this endpoint, it is ok that returned transactions are only estimates of what may actually be included in a block.