jsondb plugin transactions

The code needs to be checked and tested to ensure that operations happen atomically so that the integrity of the database is preserved as much as possible. Normal operation using the JsonDb Plugin should never put the database in an inconsistent state, however the plugin should be robust enough to such situations.

JsonDb uses the _version field of the object to handle atomicity. When writing to an object, the correct _version must be specified, otherwise the write operation will fail.