Add write time stats to celldb/db_stats.txt (#972)

Co-authored-by: SpyCheese <mikle98@yandex.ru>

validator/db/db-utils.h

@@ -0,0 +1,33 @@
+/*
+    This file is part of TON Blockchain Library.
+
+    TON Blockchain Library is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Lesser General Public License as published by
+    the Free Software Foundation, either version 2 of the License, or
+    (at your option) any later version.
+
+    TON Blockchain Library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public License
+    along with TON Blockchain Library.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#pragma once
+#include <set>
+#include <string>
+
+namespace ton::validator {
+
+class PercentileStats {
+ public:
+  void insert(double value);
+  std::string to_string() const;
+  void clear();
+
+ private:
+  std::multiset<double> values_;
+};
+
+}  // namespace ton::validator