Keep page tree current with Kids and Count so it always reflects current state.
Reposition file pointer before writing in case there was an interleaving read.
This is still not 100% correct, but it's closer.
Make sure concurrent reads and writes in different goroutines don't interfere with one another.
Keep serialization reference in xref entry while write is pending.
Use in-memory serialization reference to retrieve objects while write
is pending and clear serialization buffer when write is complete.
Also rename file.AddObjectxxx() methods to file.WriteObjectxxx().
Merge branch 'read-existing-document'