PostgreSQL in benchmarks

[n3zhov]

For now, there is no way to get pg_connection inside of ubenchmark. There are util_benchmark inside of postgres sources, but they're not linking when userver is not the root project and also PgConnection::RunStandalone class not being used depending on usage search, so the usage of this class is not clear since there are no examples. Also, functions of class marked as protected and non-static.

[apolukhin]

We'll add samples and clean up that place in a few months. If there'll be a PR from the opensource community the things would go faster and we'd be happy to review it