refactor: Move CTxMemPoolEntry class to its own module

This change nukes the policy/fees->mempool circular dependency.

Easy to review using `diff --color-moved=dimmed-zebra`.

src/test/fuzz/policy_estimator.cpp

@@ -11,6 +11,7 @@
 #include <test/fuzz/util/mempool.h>
 #include <test/util/setup_common.h>
 #include <txmempool.h>
+#include <txmempool_entry.h>
 
 #include <cstdint>
 #include <optional>