This module is going to be responsible for managing everything related
to transaction download, including txrequest, orphan transactions and
package relay. It will be responsible for managing usage of the
TxOrphanage and instructing PeerManager:
- what tx or package-related messages to send to which peer
- whether a tx or package-related message is allowed or useful
- what transactions are available to try accepting to mempool
Future commits will consolidate the interface and re-delegate
interactions from PeerManager to TxDownloadManager.
