stop using `REPEATABLE READ` everywhere

[richvdh]

currently, all postgres transactions are run in REPEATABLE READ isolation level, which is almost certainly more than we really need. The difficulty is the need to audit all of our transactions to check which actually need it.

[richvdh]

per #4473:

Maybe we could do this with an option to runInteraction which indicates whether we need to change the isolation level (defaulting to "yes"). Over time we can go through our transactions and turn the option off for transactions we are happy don't require this.

[clokep]

Maybe related to #4993?