2.0: a few hours of work

**General:**
- Few small internal simplifications
- Cache-Control header based on the maximum age acceptable (already in
- config for health checks).

**Per endpoint:**
- `/rds/all` (new): RDS instances endpoint
- `/iam/users` (new): more verbose IAM endpoint
- `/ec2/all` (new): simpler, faster EC2 instances endpoint with all machines per AZ
- `/ec2/sg` (recent and yet to be documented): restructured for per-AZ