Rework CTxMemPool::GetChildren() to not use epochs

This is likely slightly slower, but this was the last place we were using
epochs instead of sets to deduplicate, and this is only used by the RPC code
and in tests, and should not be CPU-performance critical. Eliminating this
allows us to save 8 bytes in CTxMemPoolEntry.

Co-Authored-By: Pieter Wuille <bitcoin-dev@wuille.net>

src/kernel/mempool_entry.h

@@ -12,7 +12,6 @@
 #include <policy/settings.h>
 #include <primitives/transaction.h>
 #include <txgraph.h>
-#include <util/epochguard.h>
 #include <util/overflow.h>
 
 #include <chrono>
@@ -138,7 +137,6 @@ public:
     bool GetSpendsCoinbase() const { return spendsCoinbase; }
 
     mutable size_t idx_randomized; //!< Index in mempool's txns_randomized
-    mutable Epoch::Marker m_epoch_marker; //!< epoch when last touched, useful for graph algorithms
 };
 
 using CTxMemPoolEntryRef = CTxMemPoolEntry::CTxMemPoolEntryRef;