leveldb: fix double close in MojDbLevelSeq

- Allow multyply call of close in MojDbLevelSeq
- Move MojDbLevelSeq class to separate file
- Fixed Crud test

Open-webOS-DCO-1.0-Signed-off-by: Maksym Sditanov <msditanov@luxoft.com>

Change-Id: Id91a01904c057c8b8cc74c7e732b901dd37b3dc1