Allow the tests to be configured using environment variables.

Creeated config.ts, for centralized management of user-visible
parameters.

Added instructions for developers.

Added instructions to gather some basic metrics during a benchmark run,
like WAL generation, DB-size growth.

Added ON DELETE CASCADE option to the foreign keys in Postgres schema.

Createsd a function to clone warehouse data into new warehouses.
Generating data from add_warehouses() is a bit slow, and cloning data
is about 4 times faster.

Changed the Postgres data-population scripts to create objects under
'tpcc' schema owned by 'tpcc' user.

Change Postgres communication library from `node-postgres` to
`node-postgres-pure` (that is, use `pg.js` and not `pg`). This is done
to avoid the compilation step involved in installing `node-postgres`.

Added a demo screencast to the README.