Update backup to use __wt_schema_worker. Stop LSM drops during backups.

I opened a new pull request for this because there are two outstanding issues:

1) I am no longer checking for illegal URIs in the target list (test_backup.test_illegal_objects)
2) There is a subtlety to the WiredTiger.backup file I think I'm missing. I create the file, and think the contents are the same as they were before, but when I attempt to access a table that is copied I see ```wt: table:test_backup.1: No such object exists```. To see this run the new test_backup03, but uncomment three lines from line 98.