Clarify default and allowed values of batching parameters

[hariso]

Feature description

This started in a conversation about a previous PR.

Currently, a non-zero batch size or delay is used to indicate that batching should be enabled:

conduit-connector-sdk/source_middleware.go

Line 693 in 0a04acb

if *s.config.BatchSize > 0 || *s.config.BatchDelay > 0 {

However, that seems a bit counterintuitive. From a user's point of view:

• Any value that is less than 1 doesn't make sense as a batch size.
• A batch that always has a size of 1 means that there's no batching.
• A value ≥2 means that batching should happen.

[hariso]

Also, as part of this we should make sdk.batch.size and sdk.batch.delay required together (#299 (comment))