fix: channelId on messageUpdate & honour partial objects

[austinhuang0131]

• fe325e3 adds channel ID (which is supplied by the event) onto partial messages, for those who enabled partials in ClientOptions.
• 3ffa0e7 makes partial properties actually persist in storage (otherwise isPartial is useless).
• 16b1e60 allows partial objects to be converted to full objects whenever the respective fetch methods are called.

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