Merge bitcoin/bitcoin#32827: mempool: Avoid needless vtx iteration during IBD

249889bee6 orphanage: avoid vtx iteration when no orphans (furszy)
41ad2be434 mempool: Avoid expensive loop in `removeForBlock` during IBD (Lőrinc)

Pull request description:

  During Initial Block Download, the mempool is usually empty, but `CTxMemPool::removeForBlock` is still called for every connected block where we:
  * iterate over every transaction in the block even though none will be found in the empty `mapTx`, always leaving `txs_removed_for_block` empty...
  * which is pre-allocated regardless with `40 bytes * vtx.size()`, even though it will always remain empty.

  Similarly to https://github.com/bitcoin/bitcoin/pull/32730#discussion_r2140691354, this change introduces a minor performance & memory optimization by only executing the loop if any of the affected mempool maps have any contents. The second commit is cherry-picked from there since it's related to this change as well.

ACKs for top commit:
  optout21:
    ACK 249889bee6
  glozow:
    ACK 249889bee6
  ismaelsadeeq:
    reACK 249889bee6

Tree-SHA512: 80d06ff1515164529cdc3ad21db3041bb5b2a1d4b72ba9e6884cdf40c5f1477fee7479944b8bca32a6f0bf27c4e5501fccd085f6041a2dbb101438629cfb9e4b

src/node/txorphanage.cpp

@@ -567,6 +567,8 @@ bool TxOrphanageImpl::HaveTxToReconsider(NodeId peer)
 }
 void TxOrphanageImpl::EraseForBlock(const CBlock& block)
 {
+    if (m_orphans.empty()) return;
+
     std::set<Wtxid> wtxids_to_erase;
     for (const CTransactionRef& ptx : block.vtx) {
         const CTransaction& block_tx = *ptx;