refactor: Make CTxMemPoolEntry only explicitly copyable

This has the goal of prohibiting users from accidentally creating
runtime failures, e.g. by interacting with iterator_to with a copied
entry.

CTxMemPoolEntry is already implicitly not move-constructable. So be
explicit about this and use a std::list to collect the values in the
policy_estimator fuzz test instead of a std::vector.

Co-authored-by: Anthony Towns <aj@erisian.com.au>

src/txmempool.cpp

@@ -438,7 +438,7 @@ void CTxMemPool::addUnchecked(const CTxMemPoolEntry &entry, setEntries &setAnces
     // Add to memory pool without checking anything.
     // Used by AcceptToMemoryPool(), which DOES do
     // all the appropriate checks.
-    indexed_transaction_set::iterator newit = mapTx.insert(entry).first;
+    indexed_transaction_set::iterator newit = mapTx.emplace(CTxMemPoolEntry::ExplicitCopy, entry).first;
 
     // Update transaction for any feeDelta created by PrioritiseTransaction
     CAmount delta{0};