Closely track mempool byte total. Add "getmempoolinfo" RPC.

Goal:  Gain live insight into the mempool.  Groundwork for future work
that caps mempool size.

src/txmempool.h

@@ -68,6 +68,7 @@ private:
     CMinerPolicyEstimator* minerPolicyEstimator;
 
     CFeeRate minRelayFee; // Passed to constructor to avoid dependency on main
+    uint64_t totalTxSize; // sum of all mempool tx' byte sizes
 
 public:
     mutable CCriticalSection cs;
@@ -108,6 +109,11 @@ public:
         LOCK(cs);
         return mapTx.size();
     }
+    uint64_t GetTotalTxSize()
+    {
+        LOCK(cs);
+        return totalTxSize;
+    }
 
     bool exists(uint256 hash)
     {