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.