Online garbage collection spec

Context

Related to #104 (closed), #199 (closed) and #224 (closed).

Purpose

This MR expands on the work done in gitlab-com/www-gitlab-com!60918 (merged) and documents the database queries and the processing logic required for online garbage collection, considering the schema improvements/changes being discussed in #104 (closed).

This is intended to serve as a medium for discussion/review of the overall approach, the database queries and their execution plans, as well as for persisting information for future reference.