foundation of simple db migration (#1128)

* simple db migration

* make migration slightly more functional

* migration version is just a single unsigned integer

util/src/migration/mod.rs

@@ -0,0 +1,41 @@
+// Copyright 2015, 2016 Ethcore (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity 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 General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! DB Migration module.
+
+mod db_impl;
+mod manager;
+
+#[cfg(test)]
+mod tests;
+
+pub use self::manager::{Error, Config, Manager};
+use std::collections::BTreeMap;
+
+/// Single migration.
+pub trait Migration: 'static {
+	/// Version of database after the migration.
+	fn version(&self) -> u32;
+	/// Should migrate existing object to new database.
+	/// Returns `None` if the object does not exist in new version of database.
+	fn simple_migrate(&self, key: Vec<u8>, value: Vec<u8>) -> Option<(Vec<u8>, Vec<u8>)>;
+}
+
+/// Migration destination.
+pub trait Destination {
+	/// Called on destination to commit batch of migrated entries.
+	fn commit(&mut self, batch: BTreeMap<Vec<u8>, Vec<u8>>) -> Result<(), Error>;
+}