PgSQL collector - sslmode, has_admin, connections

Commit does the following:
* add support for sslmode
* don't exhausts PgSQL connections
* don't execute query/queries which require admin privileges(configurable)

For more info, please, see https://github.com/BrightcoveOS/Diamond/issues/607

Signed-off-by: Zdenek Styblik <zdenek.styblik@icflix.com>