fix(db): increase the DNS allowed response size

Some DNS servers might reply with several IP addresses. This avoids errors like the following:

```
Failing: UPDATE `monitor` SET dns_last_result = 'Records: X' WHERE id = 1  - Data too long for column 'dns_last_result' at row 1
```

db/knex_init_db.js

@@ -86,7 +86,7 @@ async function createTables() {
         table.text("accepted_statuscodes_json").notNullable().defaultTo("[\"200-299\"]");
         table.string("dns_resolve_type", 5);
         table.string("dns_resolve_server", 255);
-        table.string("dns_last_result", 255);
+        table.string("dns_last_result", 2000);
         table.integer("retry_interval").notNullable().defaultTo(0);
         table.string("push_token", 20).defaultTo(null);
         table.text("method").notNullable().defaultTo("GET");