Query execution interface

[wangpatrick57]

Summary: Added query execution interface in env/.

Demo:
A passing unit test.

Details:

• The specific interface is the time_query() function in PostgresConn.
• I chose to have a per-query interface instead of a per-workload interface because different agents might have different ways of executing the queries of an OLAP workload. For example, Proto-X executes multiple "variations" of each query when executing a workload. A future agent might also only execute a subset of queries in a workload.
• The interface allows you to specify per-query knobs and a timeout. It returns the runtime but not the results of the query. The reason it does not return the results is so that it's compatible with Boot (which estimates the runtime but not the results).