Given the DC id, idx and point in time (in seconds),
return the temp_auth_key that should be used for the next request
(such that now < key.expires), or null if no such key exists
Store temp_auth_key for the given DC and idx, along with its expiration date (in seconds)
If key is null, the key should be deleted instead
MUST be applied immediately, without batching
Delete all stored auth keys, including both permanent and temp keys
MUST be applied immediately, without batching