_timestamp: enable mapper properties merging

Updates on the _timestamp field were silently ignored.
Now _timestamp undergoes the same merge as regular
fields. This includes exceptions if a property cannot
be changed.
"path" and "default" cannot be changed.

closes #5772
closes #6958
partially fixes #777
Not important, but couldn't this just be an array?
String[] possiblePathValues = {"some_path", "anotherPath", null};