Add serialization test for multi dimensional arrays
Remove values for BTreeSet
'DB closed' error message instead of NPE
Add Engine to check immutability of records
Merge Down Storage Class, Rename `StorageJournaled` to `StorageWriteAhead`
Fixed HTreeMap immutability; improved SnapshotEngine
Add lock-free Queue and Stack
Improve concurrency at SnapshotEngine
Fix async writer
Fix async write #2