10% of the time, when switching to a new session/cursor set, perform
a small number of reads from a checkpoint.
Merge pull request #1265 from wiredtiger/format-work

Add reads from a checkpoint.
Don't read from checkpoints if we're single-threaded and doing checks
against a BDB copy of the object, the checkpoint might not match the
BDB data.
Skip checkpoint reads from LSM: they aren't supported.