Allow adding headroom to the connection pool

What does this MR do?

This allows setting the DB_POOL_HEADROOM env variable to add a number of connections to the pool.

In normal situations, these connections should not be used, but it allows us to investigate the problem without degrading the service.

We'll set this to an acceptable number for GitLab.com, and alert when we start using those spare connections.

Required for gitlab-com/gl-infra/scalability#401 (closed) Since we're already using connections we weren't expecting: gitlab-com/gl-infra/scalability#409 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required): We explicitly don't document this, because it should not be needed.
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done