Renamed helper method to starte with 'Write',

instead of starting with 'Add', in order to make it clearer that this
method actually writes documents to the underlying storage.
Extracted a helper method.
Added extra arguments to WriteEntryToNewPage,

even though they aren't needed, because it makes it clear that the
signature of WriteEntryToNewPage is the same as WriteEntryToExistingPage.

(At this point, the Verify build configuration fails because of those
unused arguments.)
Introduced internal AppendContext class

and passed it as an argument to Write methods, although it's not yet used.
Used LastLink boolean values from context.
Removed unused flag arguments.
Used Now value from context instead of argument.
Moved usage of lastPage to context,

and removed unused argument.
Moved usage of index to context,

and removed unused argument.