[Feature] microbatches should default run in parallel according to threads

[graciegoheen]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

For faster processing of microbatch models, we should by default run the batches in parallel. We are assuming batches can be run independently.

We could additionally:

• have a config to optionally run batches sequentially for specific use cases
• automatically detect which batches can not be run in parallel

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response