Build session objects from environment variables

[sbellware]

Workaround for the PG gem's lack of support for this later Postgres feature. The single environment variable connection string is now the standard on Heroku, apparently. We have to consider the value of even supporting Heroku.

[samstickland]

Wouldn't this also apply to any sort of environment where the instances are designed to be as stateless as possible, and also collect their configuration via environment variables? I'm not sure this is Heroku specific parsae

[sbellware]

@samstickland I'm not sure what you mean by "stateless". Service components are almost never stateless. Nonetheless, the capability implied by this change would allow any environment that uses environment variables to configure a Postgres connection via the PG gem using an environment variable to hold the connection string.

[sbellware]

Can't validate this feature. Will defer for later consideration.