rpc: make gettxoutsettinfo run lock-free

For leveldb "An iterator operates on a snapshot of the database taken
when the iterator is created". This means that it is unnecessary to
lock out other threads while computing statistics, and neither to hold
cs_main for the whole time. Let the thread run free.

src/txdb.cpp

@@ -225,7 +225,10 @@ bool CCoinsViewDB::GetStats(CCoinsStats &stats) const {
             return error("%s: Deserialize or I/O error - %s", __func__, e.what());
         }
     }
-    stats.nHeight = mapBlockIndex.find(GetBestBlock())->second->nHeight;
+    {
+        LOCK(cs_main);
+        stats.nHeight = mapBlockIndex.find(stats.hashBlock)->second->nHeight;
+    }
     stats.hashSerialized = ss.GetHash();
     stats.nTotalAmount = nTotalAmount;
     return true;