When `sdk.batch.size` is specified but `sdk.batch.delay` isn't, the SDK will wait indefinitely for the batch size to fill up. We should make the user aware of that (docs, and a warning /failed validation). Tasks: - [x] Update docs (done as part of https://github.com/ConduitIO/conduit-site/pull/298) - [x] Log a warning https://github.com/ConduitIO/conduit-connector-sdk/pull/309 - [ ] Remove 0 as default value.