Add Sleep benchmark and dialects for MySQL and Postgres

[mbutrovich]

WIP PR. Adds a new benchmark (dialects for Postgres and MySQL so far) to just sleep. Questtions:

• Not sure how to write the generic query for this since it's not in the SQL standard. Right now it's just a ; like in NoOp.
• Do we need a DDL file? I borrowed the one from NoOp.

[apavlo]

@mbutrovich Are you going to add XML config options to control # of queries per txn and amount of sleep per query?