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

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

src/rpcblockchain.cpp

@@ -531,3 +531,27 @@ Value getchaintips(const Array& params, bool fHelp)
 
     return res;
 }
+
+Value getmempoolinfo(const Array& params, bool fHelp)
+{
+    if (fHelp || params.size() != 0)
+        throw runtime_error(
+            "getmempoolinfo\n"
+            "\nReturns details on the active state of the TX memory pool.\n"
+            "\nResult:\n"
+            "{\n"
+            "  \"size\": xxxxx                (numeric) Current tx count\n"
+            "  \"bytes\": xxxxx               (numeric) Sum of all tx sizes\n"
+            "}\n"
+            "\nExamples:\n"
+            + HelpExampleCli("getmempoolinfo", "")
+            + HelpExampleRpc("getmempoolinfo", "")
+        );
+
+    Object ret;
+    ret.push_back(Pair("size", (int64_t) mempool.size()));
+    ret.push_back(Pair("bytes", (int64_t) mempool.GetTotalTxSize()));
+
+    return ret;
+}
+