CreDocument: fix document loading interferences (#4349)

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

frontend/apps/reader/readerui.lua

@@ -263,6 +263,11 @@ function ReaderUI:init()
             document = self.document,
         })
     else
+        -- load crengine default settings (from cr3.ini, some of these
+        -- will be overriden by our settings by some reader modules below)
+        if self.document.setupDefaultView then
+            self.document:setupDefaultView()
+        end
         -- make sure we render document first before calling any callback
         self:registerPostInitCallback(function()
             if not self.document:loadDocument() then