Fix read receipt sending behaviour around thread roots (#3600)
* Fix read receipt sending behaviour around thread roots
* Update src/client.ts
Co-authored-by: Eric Eastwood <erice@element.io>
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Co-authored-by: Eric Eastwood <erice@element.io>
(cherry picked from commit b05f933d833ef6c9b2a8ff7a8b27723b682bcce6)