Code cleanup in LeveldbRMStateStore

Some things can be improved:

• throws Exception declaration can be removed from LeveldbRMStateStore.initInternal method 
• key variable is redundant in LeveldbRMStateStore.dbStoreVersion
• try can use automatic Resource management in LeveldbRMStateStore.loadReservationState/loadRMDTSecretManagerKeys/loadRMDTSecretManagerTokens/loadRMApps/... etc
• there were some methods which were copied to LeveldbConfigurationStore (ie: openDatabase, storeVersion, loadVersion, CompactionTimerClass nested class), a helper class could be created to reduce the duplicated code
• Any other cleanup