Relay Transactions
Comparatively to the rest, relaying unconfirmed transactions is not too difficult. Our peers advertise their inventory
by sending Inv messages. We already use them to detect new blocks. Now we also need to use the transaction inventory.
We keep a TxPool which is a hash map of transaction hashes to transaction. Initially the pool is empty. When we
receive an Inv message, we look at the pool and see if we already know about this message. If so, nothing else happens.
If it's a new transaction, we insert an entry pointing to None. This indicates that we know about the transaction
but we haven't got the transaction itself. Then we send a GetData message to request it.
Once we get the transaction, we use our checkTx function to make sure that it is valid and then send out a InvEntry
message to all our peer actors. They will buffer the InvEntry together for 1 second before sending out an Inv.
This way we won't send an Inv message with a single transaction in it.
Whenever we receive a new block, we clear the pool. The TxPool is not the same memory pool as Bitcoin Core. We are not
mining so we don't care about having all the transactions. Our rule is that if a transaction isn't confirmed in one block,
we ditch it.
