3f987218 introduced new API for fast finding of marks to a specific text node,
so use that (code itself based on cb46aaf2).
This makes mailmerge faster (since it can create a huge document, and especially
with the change to use UNO bookmarks to mark starts of MM documents inside
the larger single document there can be a large number of marks).

Change-Id: I30ec69acf423e9a62fae5f5492ed8744cb727a56