Todo: double-check graceful shutdown flow

See also https://github.com/flashbots/mev-boost-relay/blob/main/docs/docs/20230619-startup-shutdown-zero-downtime-deployments.md

Review httpserver shutdown in light of that post.

Start by reading the linked zero-downtime deployments document and then locate the repository's httpserver shutdown flow. Compare the current behavior with the document's guidance; done means the shutdown flow has been reviewed and any required changes are covered by appropriate validation.

Written by the indexing model from the issue text.