Bug 1826206 - Require nsISerialEventTarget for RetargetDeliveryTo, r=necko-reviewers,valentin, a=dmeehan

This avoids potential issues where multiple OnDataAvailable callbacks or
similar could theoretically be called concurrently on different
StreamTransportService threads when targeting the STS - these cases will
now target a TaskQueue on the STS instead, structurally ensuring serial
execution.

Differential Revision: https://phabricator.services.mozilla.com/D179984