Switch the update serialization function to an atomic instruction instead
of acquiring/releasing a spinlock.
Serialize threads of control doing the "obsolete WT_UPDATE" check.
The code inside __col_append_serial_func() is the same as the code in
__insert_serial_func(), now that the functions are inlined, might as
well only have a single copy.
Merge pull request #723 from wiredtiger/atomic-update

Switch the update serialization function to an atomic instruction instead of acquiring/releasing a spinlock.