Do direct I/O reads on checkpoints to reduce the VM pressure of LSM workloads.
Merge branch 'develop' into checkpoint-directio
Merge branch 'develop' into checkpoint-directio
Do buffer alignment checks when reading from checkpoints.

Update test_checkpoint01 to use valid buffer sizes.
Update test/salvage to configure the right buffer size for direct io.
Don't automatically enable direct I/O on checkpoints.

Using direct I/O requires careful selection of page sizes, and doesn't
always improve performance. It's not worth enabling it automatically.
Revert unnecessary test changes.
Update documentation for direct_io configuration.
Merge pull request #847 from wiredtiger/checkpoint-directio

Do direct I/O reads on checkpoints to reduce the VM pressure of LSM workloads