_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
This question actually bubbled up a bug in the doXContent on TimestampFieldMapper. Fixed in "fix toXContent, build correct "index" value"