Change dump_db isolation level to "REPEATABLE READ"

This addresses the following error:

```
sql_error_code = 0A000 ERROR:  cannot use serializable mode in a hot standby
sql_error_code = 0A000 HINT:  You can use REPEATABLE READ instead.
sql_error_code = 0A000 STATEMENT:  BEGIN ISOLATION LEVEL SERIALIZABLE READ ONLY DEFERRABLE;

```

Because our application does not rely on any other `SERIALIZABLE`
transactions, it should be fine to relax the isoluation level for
database dumps to `REPEATABLE READ`.

`DEFERRABLE` is dropped because it is only meaningful when combined
with `SERIALIZABLE`.

Author: jtgeibel

src/tasks/dump_db/dump-export.sql.hbs

@@ -1,4 +1,4 @@
-BEGIN ISOLATION LEVEL SERIALIZABLE READ ONLY DEFERRABLE;
+BEGIN ISOLATION LEVEL REPEATABLE READ, READ ONLY;
 {{~#each tables}}
 {{~#if this.filter}}
     \copy (