When a main document is opened for displaying, some other
document openings (for getting metadata or cover image)
could affect the main document.
Split some code from CreDocument:init() into another new
method CreDocument:setupDefaultView(), that will only be
called by ReaderUI when opening the main document (and not
by these other openings like Book inforation, View cover...)
Also speed up some of these other openings (Search, Screensaver)
by using doc:loadDocument(false) to load only metadata and
avoid parsing the HTML.
Details in https://github.com/koreader/koreader/issues/4346#issuecomment-440036496
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