Switch from environment variable to behavior flag for gating microbatch functionality

[QMalcolm]

resolves #327

Problem

We're currently gating microbatch functionality via environment variables, and we should instead use a behavior flag

Solution

Switch to behavior flag for gating microbatch functionality

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

[QMalcolm]

So it turns out, we actually need to do MUCH less. I'm going to drop a bunch of commits... Like half of them. And then also squash some of them. And then make some changes to simplify the tests further.