Add lock/unlock test. Fix bug with read only and seek
Add lock/unlock test. Fix bug with read only and seek

FootballAddicts/SPTPersistentCache,FootballAddicts/SPTPersistentCache,chrisbtreats/SPTPersistentCache,FootballAddicts/SPTPersistentCache,iOSCowboy/SPTPersistentCache,chrisbtreats/SPTPersistentCache,spotify/SPTPersistentCache,spotify/SPTPersistentCache,chrisbtreats/SPTPersistentCache,FootballAddicts/SPTPersistentCache,iOSCowboy/SPTPersistentCache,chrisbtreats/SPTPersistentCache,spotify/SPTPersistentCache,iOSCowboy/SPTPersistentCache