[prep/test] have TxOrphanage remember its own limits in LimitOrphans

Move towards a model where TxOrphanage is initialized with limits that
it remembers throughout its lifetime.
Remove the param. Limiting by number of unique orphans will be removed
in a later commit.
Now that -maxorphantx is gone, this does not change the node behavior.
The parameter is only used in tests.

src/net_processing.cpp

@@ -1925,7 +1925,7 @@ PeerManagerImpl::PeerManagerImpl(CConnman& connman, AddrMan& addrman,
       m_banman(banman),
       m_chainman(chainman),
       m_mempool(pool),
-      m_txdownloadman(node::TxDownloadOptions{pool, m_rng, opts.max_orphan_txs, opts.deterministic_rng}),
+      m_txdownloadman(node::TxDownloadOptions{pool, m_rng, opts.deterministic_rng}),
       m_warnings{warnings},
       m_opts{opts}
 {