mempool: Introduce (still-unused) MemPoolLimits

They live as a CTxMemPool member.

[META] These limits will be used in subsequent commits to replace calls
       to gArgs.

src/txmempool.cpp

@@ -455,7 +455,8 @@ CTxMemPool::CTxMemPool(const Options& opts)
     : m_check_ratio{opts.check_ratio},
       minerPolicyEstimator{opts.estimator},
       m_max_size_bytes{opts.max_size_bytes},
-      m_expiry{opts.expiry}
+      m_expiry{opts.expiry},
+      m_limits{opts.limits}
 {
     _clear(); //lock free clear
 }