[txdownload] add read-only reference to mempool

This will become necessary in later commits that query mempool. We also
introduce the TxDownloadOptions in this commit to make the later diff
easier to review.

src/net_processing.cpp

@@ -2000,6 +2000,7 @@ PeerManagerImpl::PeerManagerImpl(CConnman& connman, AddrMan& addrman,
       m_banman(banman),
       m_chainman(chainman),
       m_mempool(pool),
+      m_txdownloadman(node::TxDownloadOptions{pool}),
       m_warnings{warnings},
       m_opts{opts}
 {