Fix getchaintxstats()

1. Calculate nblocks more adaptive.
   If not specify nblocks-parameter, illegal parameter error
   will happen when target block height is below blocks for 1 month.
   To avoid this error, set default nblocks to
   min(blocks for 1 month, target block's height - 1)
   And allowing 0 so that this RPC works good even if target block is
   genesis block or 1st block.
2. Correct error message.
   nblocks accepts [0 .. block's height -1] . so fix as following:
   "Invalid block count: should be between 0 and the block's height - 1"
3. Add check 0-divide.
   If nTimeDiff = 0 then returns {... "txrate":} and
   bitcoin-cli cannot handle the response.
   To avoid this error, do not return "txrate" if nTimeDiff = 0.
4. Add following 3 elements to the return object.
   1) 'window_block_count' : Size of the window in number of blocks.
   2) 'window_tx_count' : The number of transactions in the window.
   3) 'window_interval' : The elapsed time in the window.
   They clarify how 'txrate' is calculated. 2) and 3) are returned
   only if 'window_block_count' is a positive value.
5. Improve help text for 'time' as following.
   'The timestamp for the final block in the window in UNIX format.

src/rpc/blockchain.cpp

@@ -1477,9 +1477,12 @@ UniValue getchaintxstats(const JSONRPCRequest& request)
             "2. \"blockhash\"  (string, optional) The hash of the block that ends the window.\n"
             "\nResult:\n"
             "{\n"
-            "  \"time\": xxxxx,        (numeric) The timestamp for the statistics in UNIX format.\n"
+            "  \"time\": xxxxx,                (numeric) The timestamp for the final block in the window in UNIX format.\n"
             "  \"txcount\": xxxxx,             (numeric) The total number of transactions in the chain up to that point.\n"
-            "  \"txrate\": x.xx,       (numeric) The average rate of transactions per second in the window.\n"
+            "  \"window_block_count\": xxxxx,  (numeric) Size of the window in number of blocks.\n"
+            "  \"window_tx_count\": xxxxx,     (numeric) The number of transactions in the window. Only returned if \"window_block_count\" is > 0.\n"
+            "  \"window_interval\": xxxxx,     (numeric) The elapsed time in the window in seconds. Only returned if \"window_block_count\" is > 0.\n"
+            "  \"txrate\": x.xx,               (numeric) The average rate of transactions per second in the window. Only returned if \"window_interval\" is > 0.\n"
             "}\n"
             "\nExamples:\n"
             + HelpExampleCli("getchaintxstats", "")
@@ -1489,10 +1492,6 @@ UniValue getchaintxstats(const JSONRPCRequest& request)
     const CBlockIndex* pindex;
     int blockcount = 30 * 24 * 60 * 60 / Params().GetConsensus().nPowTargetSpacing; // By default: 1 month
 
-    if (!request.params[0].isNull()) {
-        blockcount = request.params[0].get_int();
-    }
-
     bool havehash = !request.params[1].isNull();
     uint256 hash;
     if (havehash) {
@@ -1515,8 +1514,14 @@ UniValue getchaintxstats(const JSONRPCRequest& request)
         }
     }
 
-    if (blockcount < 1 || blockcount >= pindex->nHeight) {
-        throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid block count: should be between 1 and the block's height");
+    if (request.params[0].isNull()) {
+        blockcount = std::max(0, std::min(blockcount, pindex->nHeight - 1));
+    } else {
+        blockcount = request.params[0].get_int();
+
+        if (blockcount < 0 || (blockcount > 0 && blockcount >= pindex->nHeight)) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid block count: should be between 0 and the block's height - 1");
+        }
     }
 
     const CBlockIndex* pindexPast = pindex->GetAncestor(pindex->nHeight - blockcount);
@@ -1526,7 +1531,14 @@ UniValue getchaintxstats(const JSONRPCRequest& request)
     UniValue ret(UniValue::VOBJ);
     ret.push_back(Pair("time", (int64_t)pindex->nTime));
     ret.push_back(Pair("txcount", (int64_t)pindex->nChainTx));
+    ret.push_back(Pair("window_block_count", blockcount));
+    if (blockcount > 0) {
+        ret.push_back(Pair("window_tx_count", nTxDiff));
+        ret.push_back(Pair("window_interval", nTimeDiff));
+        if (nTimeDiff > 0) {
             ret.push_back(Pair("txrate", ((double)nTxDiff) / nTimeDiff));
+        }
+    }
 
     return ret;
 }