Simplify test/format timestamp handling.

The current implementation of test/format tracks all active timestamps in order to be able to choose a legal read timestamp for operations. That tracking is complex, and with the addition of prepare support requires co-ordinating timestamp allocation via a lock.

test/format could instead query WiredTiger via the WT_CONNECTION::query_timestamp() API to retrieve a legal read timestamp, and then it wouldn't need the complex co-ordination code.