Change transaction IDs to 64-bits, remove code that deals with wrapping.
Back out one part of the previous change: new snapshot transactions should get an ordinary snapshot.
Update spelling list.
whitespace
Merge pull request #568 from wiredtiger/64bit-txnids

Change transaction IDs to 64-bits, remove code that deals with wrapping.
whitespace, fix a comment.
Fold some too-long lines.