NewBlockFetcher creates a block fetcher to retrieve blocks based on hash announcements.

NewTxFetcher creates a transaction fetcher to retrieve transaction based on hash announcements.

NewTxFetcherForTests is a testing method to mock out the realtime clock with a simulated version and the internal randomness with a deterministic one.

BlockFetcher is responsible for accumulating block announcements from various peers and scheduling them for retrieval.

TxFetcher is responsible for retrieving new transaction based on announcements.

HeaderRetrievalFn is a callback type for retrieving a header from the local chain.