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 (mixpanel/mixpanel-swift#600) from the Swift project to this project.