Issue #141: Introduce state system for environment-specific,  non-configuration settings.

PR for https://github.com/backdrop/backdrop-issues/issues/141.

This PR includes conversion of the following variables to the state system:

- cron_key
- cron_last
- css_js_query_string
- install_task
- install_time
- maintenance_mode

Overall, this matches the basic functionality of D8 but with a different approach (simple procedural code instead of a "service" with a class hierarchy). However this currently means 2 additional SQL queries per page (for maintenance_mode and css_js_query_string).