Add Egress benchmark

[mbutrovich]

Not ready for review yet.

This adds a configurable Egress benchmark to return result rows based on two parameters: egress_tuple_bytes and egress_num_tuples. Since UDF syntax varies by DBMS, this only includes a dialect and ddl file for Postgres. Hopefully folks can add more if this merges.

[mbutrovich]

Apparently the ScriptRunner class we use to run ddl .sql files can't handle plpgsql. I have to imagine there's a better way to load and run a .sql file over JDBC than a Java class from 2004, but I'll hack around it locally for now by just issuing a psql command with the .sql file.