feat: do not store channelData in thread state

[myandrienko]

Cleaning up types a bit:

1. State when Thread is initialized without an underlying Channel is not very useful. Now threads always have a channel.
2. Storing channelData separately is excessive, we have it in channel.
3. staggeredRead removed for now, it's supposed to make a triumphant return at some point.

This will break ThreadProvider in React SDK, fixed in a follow-up PR.

[github-actions]

Size Change: -787 B (-0.18%)

Total Size: 425 kB

Filename	Size	Change
dist/browser.es.js	92.5 kB	-168 B (-0.18%)
dist/browser.full-bundle.min.js	52.7 kB	-83 B (-0.16%)
dist/browser.js	93.6 kB	-180 B (-0.19%)
dist/index.es.js	92.6 kB	-171 B (-0.18%)
dist/index.js	93.7 kB	-185 B (-0.2%)

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