fix(gc): improve handling of database errors and review postponing

Related to #440 (closed). Please see the issue for a description of the problem and the solution we're after in this MR.

Database review

This MR adds one new query:

SELECT
    top_level_namespace_id,
    repository_id,
    manifest_id,
    review_after,
    review_count
FROM
    gc_manifest_review_queue
WHERE
    top_level_namespace_id = $1
    AND repository_id = $2
    AND manifest_id = $3
FOR UPDATE

                                                                         QUERY PLAN                                                                          
-------------------------------------------------------------------------------------------------------------------------------------------------------------
 LockRows  (cost=0.15..8.18 rows=1 width=42) (actual time=4.866..4.873 rows=1 loops=1)
   ->  Index Scan using pk_gc_manifest_review_queue on gc_manifest_review_queue  (cost=0.15..8.17 rows=1 width=42) (actual time=3.657..3.663 rows=1 loops=1)
         Index Cond: ((top_level_namespace_id = 34) AND (repository_id = 48) AND (manifest_id = 7))
 Planning Time: 22.349 ms
 Execution Time: 7.091 ms
(5 rows)