MXThrottler: MXify it and fix timers stacking
MXThrottler: MXify it and fix timers stacking

When throttling, the throttle operation created a dispatch_after every time it  was called. All blocks were indeed  called at the end.
matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk,matrix-org/matrix-ios-sdk