Workgen changes - Add session config at Thread level

Workgen fails when using more prepare_timestamp operation threads in Workgen.

 [1586857531:330755][32283:0x7fffbeffd700], file:test0.wt, WT_SESSION.begin_transaction: __split_multi_inmem, 1468: upd != supd->onpage_upd

[1586857531:332100][32283:0x7fffbeffd700], file:test0.wt, WT_SESSION.begin_transaction: __wt_abort, 28: aborting WiredTiger library

Steps to reproduce :

Change the following configurations in example_prepare.py

• Set write_txn.transaction.use_prepare_timestamp = True
• Increase the number of update threads ex: update_txn = txn(opupdate * 30, 'isolation=snapshot')