[iOS] Ensure consistent willChange/didChange notifications

Coalescing willChange/didChange notifications means keeping track of how many gestures are currently in progress; a boolean isn’t enough to track this state. This change refactors the gesture recognizers, making them more consistent with each other and more consistent in the case where more than one of them has fired. It also explicitly cancels transitions before all programmatic viewport-modifying methods, since mbgl only does so when animating.

Fixes #2313, fixes #2379, fixes #3062.

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