Two bug fixes: First, handle close checkpoints files and so can discard
underlying checkpoint files, it needs to check if a hot backup is in
progress.  Second, checkpoint needs to lock out the start of a hot backup
once it checks the hot-backup flag, otherwise the hot backup could start
while the checkpoint is still deleting file blocks.
Merge branch 'develop' into hot-backup-race
Merge branch 'develop' into hot-backup-race
Merge pull request #515 from wiredtiger/hot-backup-race

Fix a bug where checkpoints are being deleted while hot backups are running.