Bulk-load needs to lock out other cursor operations

Bulk-load needs to lock out reads, inserts, and whatever else, I think (if you do an insert into a tree that's being bulk-loaded, no good will come from that).

We could do that in the lower levels, but maybe bulk-load should be like verify & salvage, that is, the upper level just fails the op if there are other cursors running, and the op locks out other cursors.