Always set up next pointers before calling insert_serial.  Never search inside the serial lock: if the configured insert isn't in the right place, restart the operation.

Follows on from the changes in #718
Simplify the col_append serial function -- make it closer to a normal insert.
Fix the test for empty skiplists.

refs #719
minor wordsmithing on a couple of comments.
more minor comment wordsmithing, no real changes.
Merge pull request #719 from wiredtiger/serial-simpify

More simplification of serial functions