Faster remote room joins (worker mode): do not populate external hosts-in-room...

Faster remote room joins (worker mode): do not populate external hosts-in-room cache when sending events as this requires blocking for full state. (#14749)

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>
Co-authored-by: Sean Quah <seanq@matrix.org>