WriteAheadLog; updated StorageDriver
Async Update & Fetch
Write-ahead log for updates
Moved processing queue from StorageDriver to Engine
Rewrite of the Engine.process() mechanism
Fixed document modification, added unit test for modification
Renamed FlexImutStorage to FlexStorage
Fix node.js API; Improve unit tests

The node.js API contained a serious bug when running
two clients in the same process.
Remove debug messages