fix: Add maxQueuedEventCount parameter to trim events in storage if over a limit

[crleona]

Summary

Adds a configuration parameter, maxQueuedEventCount, intended to control the maximum number of events we store on device. Set to a negative value to have no max count.

Every time we initialize Amplitude, we check maxQueuedEventCount. If set to a value greater than zero, we will delete all but the most recent maxQueuedEventCount events. Note that we will never attempt to split or otherwise rewrite a block during this process, so we may store an actual max event count of maxQueuedEventCount + flushQueueSize - 1. We also never will clear storage during the lifetime of the Amplitude instance, so more than maxQueuedEventCount events may be queued on the device until the next time Amplitude is run.

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