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/validation.cpp

@@ -42,6 +42,7 @@
 #include <tinyformat.h>
 #include <txdb.h>
 #include <txmempool.h>
+#include <txmempool_entry.h>
 #include <uint256.h>
 #include <undo.h>
 #include <util/check.h> // For NDEBUG compile time check