support retries in [[create]] & [[setup]] steps

[zeroXbrock]

If a contract deployment or setup step fails, the whole scenario deployment has to be scrapped; when you run it again and it works, the previously-deployed contracts will be effectively forgotten (though they're still in the DB).

High-level goal: When running contender setup, check the DB to see if all create/setup transactions were sent, and if not, resume the previous deployment and save to DB accordingly.

Technical details:

For [[create]] steps, you can look up the required entries by name in the named_txs table. This makes it simple to find the contract deployments that haven't been executed: just count the number of entries in named_txs for each name in the TestScenario's create steps. The steps that haven't deployed will have one less entry in the DB than the ones that have been deployed.

Achieving the "resume" effect for setup txs will be slightly more involved. [[setup]] steps don't require names, so we'll have to save new info to the DB. A new table may be in order:

scenario_setup

key	type
id	SERIAL INT PRIMARY KEY
scenario_name	VARCHAR UNIQUE
successful_txs	INT

For each successful setup tx sent, increment successful_txs in the DB by 1. Then, to find where to resume after a failed run, compare successful_txs for this scenario to the number of [[setup]] directives in the scenario.