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.
If restored a tombstone WT_UPDATE after splitting, calculate the correct size.
Merge branch 'develop' into basecfg-version
The upgrade configuration string is the same string to which we're
appending new configuration options, clarify that by only using one
parameter.
Add some comments explaining the different wiredtiger_open configuration
string variants.