bulk-load state changes can race

Related to WT-3193, WT-3199: the bulk-load state change (where multiple threads of control turn off the possibility of bulk-load in a tree), has always been able to race, but it's potentially dangerous now that turning off bulk-load involves calling __wt_evict_file_exclusive_off(). In the current tree, there's a diagnostic-only test that might drop core because of this race.