Allow for managing separate client objects?

[sckott]

Right now on the dbs branch we allow only one client instance for each of Redshift and RDS.

One needs a new client for either of those services only if there are different credentials; if credentials don't change then the same client can be reused.

How to do this?

1. Tell users to open up separate R sessions, one for each set of unique credentials.
2. Require passing in client object into each function that interacts with Redshift/RDS.