Mark Transaction as Read-Only When Executing Single-Statement SELECT

[apavlo]

If we get SELECT query in a single-statement txn, we should mark it as 'read-only'. This will avoid conflicts with other txns that are running at the same time.

I think the only place that we need to change is TrafficCop::ExecuteHelper():

https://github.com/cmu-db/peloton/blob/master/src/traffic_cop/traffic_cop.cpp#L163

I'm not sure how we can write a test case to prove that this is working correctly. I don't know whether we have test cases for running single-stmt txns in the TrafficCop and then being able to see the TransactionContext after the txn commits.

[apavlo]

I will assign this task to @lmwnshn