Add indexes and enable WAL mode on SQLite

[weibel]

Integration Method: CocoaPods Xcode Version: 14.3 Library Version: 5.0.2 Platform: iOS Language: Swift & Objective-C Description: Mixpanel uses close to 50% of our apps disk writes Expected Behavior: Write less to disk

Apply this fix (https://github.com/mixpanel/mixpanel-swift/pull/600) from the Swift project to this project.