Limit the size of aggregated WebXDC update

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Currently update sending may be delayed due to rate limits and later merged into large messages. This is undesirable for apps that want to send large files over WebXDC updates, e.g. application updates sent by the store bot or large files imported via importFiles, because the message with aggregated update may be too large for actual sending and hit the provider limit or require multiple attempts on a flaky SMTP connection.

Updates should not be aggregated to pending updates if the size of aggregated update will exceed some limit, e.g. 100 kB. Adding an option for configuring this limit and splitting updates that are themselves larger than this limit is out of scope of this issue.