Begin supplying a batch_id in the mode context for batches given to adapters

[QMalcolm]

In dbt-labs/dbt-adapters#360, we're trying to ensure each batch ends up making a distinct temp_relation in the data warehouse (as they are currently using the same temp_relation which is bad). In order for each batch to have a different temp_relation, we need to ensure that there is some sort of batch identifier information in the model context. We believe it would be best to start supplying a batch_id which for a given microbatch model is guaranteed to be unique, but is also human-readable to aid in debugging.