[PATCH] pgbench: add multiconnect option

-hackers,

This patch adds the concept of "multiconnect" to pgbench (better
terminology welcome). The basic idea here is to allow connections made
with pgbench to use different auth values or connect to multiple
databases. We implement this using a user-provided PGSERVICEFILE and
choosing a PGSERVICE from this based on a number of strategies.
(Currently the only supported strategies are round robin or random.)

There is definite room for improvement here; at the very least, teaching
`pgbench -i` about all of the distinct DBs referenced in this service
file would ensure that initialization works as expected in all places.
For now, we are punting initialization to the user in this version of
the patch if using more that one database in the given service file.

Best,

David