Cancelled verification requests flash up after opening app

[dbkr]

• Have an ios device logged in with element closed
• Send a verification request to it (eg. log in with another device)
• Cancel the verification request
• Open element on ios

A verification request dialog appears briefly, then disappears.

Similar to element-hq/element-android#3563 on android, but less severe.

[ShadowJonathan]

I think this is because all to-device messages are sent to the device in batches? (And/or the device is firing event handlers on each individual one in sequence)

This should be easy to fix by iterating /sync with 0 timeout until the server returns no more to-device, handling them all in one batch, and only then displaying any potential "leftovers" to the user.