Stop running checkpoint tests after a failure.
Simplify test/checkpoint: use integer keys in row stores to avoid different code paths for ROW vs COL.  Ensure all tables are updated to the same value.
If there is a value mismatch in a checkpoint, do lookups in the original tree to see whether it is okay there.
Merge pull request #990 from wiredtiger/test-checkpoint-mjc

Simplify test/checkpoint