Super property persistence fix

Improves reliability of archiving and flushing on background.

Time bug fix

We were setting the time on events and records inside async blocks. This resulted in incorrect, delayed time values. Time is now set at the moment track: or any of the Mixpanel People property methods, e.g., set:, append:, etc., are called.

64-bit compatibilty

Adds some required casting for 64-bit builds.