Update for testing client handshake.
Update for testing client handshake.

See https://jira.mongodb.org/browse/DRIVERS-304. MongoDB drivers now
add a "client" subdocument to identify themselves to servers during
handshake, so we can't use "client" as a keyword arg to Request.

Rename it "_client", and rename "server" to "_server" for consistency.

ajdavis/mongo-mockup-db