Allow configurable online GC review delays
This is the first part of #299 (closed). It includes all the required database DDL changes (new table and updated functions).
See #299 (comment 515178331) for the rationale behind these changes, and !552 (diffs) for an explanation of the solution implemented here (slight differences when compared with the linked thread).
Related to #299 (closed).
Edited by João Pereira