make 'connection pool' example more conventional

-- remove unused imports
-- clean up syntax issues
-- use more pythonistic routines (e.g., list and dictionary
comprehension)

It still doesn't work with Python 3.

Change-Id: I95cfbac22f5f3a8c5bdf9b187b8b3e89f1d7be3a
Reviewed-on: http://review.couchbase.org/29165
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>