Fix a race between reconciliation and freeing obsolete updates on a page.  Now that reconciliation scans to the end of update lists, it is not safe for it to run concurrently with freeing obsolete updates.  Use an atomic page flag rather than a mutex to protect access: there is no real need to block update operations if there is contention, this is just trimming old memory.
Merge pull request #946 from wiredtiger/rec-obsolete-race

Fix a race between reconciliation and freeing obsolete updates on a page.