Wrap query in transaction

This alleviates current locking issues.

Related to swh/infra/sysadm-environment#4228 (closed)

Test Plan

swh/infra/sysadm-environment#4228 (closed)

Corrupt objects are popping up now (and growing fast):

Tue 31 May 2022 11:01:14 AM CEST (every 2s)

+-------------------------------+-------+
|              now              | count |
+-------------------------------+-------+
| 2022-05-31 09:01:14.233113+00 |  1620 |
+-------------------------------+-------+
(1 row)
+-------------------------------+-------+
|              now              | count |
+-------------------------------+-------+
| 2022-05-31 09:01:54.394741+00 |  3233 |
+-------------------------------+-------+
(1 row)

Time: 6.463 ms

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Migrated from D7914 (view on Phabricator)