eth/fetcher: remove alternates, announced maps

[healthykim]

This PR removes the alternates and announced maps in tx_fetcher.

• The alternates map is used only to check whether a transaction is in stage 2 (queued), and the announced map is used only to check whether a transaction is in stage 3 (fetching).
• However, regardless of the stage, transactions eventually go through a similar process (being marked in the announces map). So maintaining two separate maps seems unnecessary.
• Instead, we can iterate over the announces map, which is shared across stage 2 and stage 3, to perform the same check.

Given that the peer set is usually small (e.g., 50 by default), this iteration is expected to be fast.