refactor: mempool: add MemPoolLimits::NoLimits()

There are quite a few places in the codebase that require us to
construct a CTxMemPool without limits on ancestors and descendants.
This helper function allows us to get rid of all that duplication.

src/kernel/mempool_limits.h

@@ -24,6 +24,15 @@ struct MemPoolLimits {
     int64_t descendant_count{DEFAULT_DESCENDANT_LIMIT};
     //! The maximum allowed size in virtual bytes of an entry and its descendants within a package.
     int64_t descendant_size_vbytes{DEFAULT_DESCENDANT_SIZE_LIMIT_KVB * 1'000};
+
+    /**
+     * @return MemPoolLimits with all the limits set to the maximum
+     */
+    static constexpr MemPoolLimits NoLimits()
+    {
+        int64_t no_limit{std::numeric_limits<int64_t>::max()};
+        return {no_limit, no_limit, no_limit, no_limit};
+    }
 };
 } // namespace kernel