Turn on partial parser by default

[leahwicz]

Pre-req before turning on:

• Trigger full reparse if changes to widely used built-in macros #3810
• Env vars + partial parsing #3885
• Jinja functions remain static in YAML files on RPC until modified #2330 (comment)

[leahwicz]

Broke this out from the over-arching story: #3502

[jtcohen6]

I added one more pre-req above, based on the comments I'd left in #3502.

My current feelings about these pre-reqs:

• I believe we can and should resolve Trigger full reparse if changes to widely used built-in macros #3810
• I could go any way on env vars. @gshank I'd appreciate your take on the relative complexity of those options.
• I'm leaning toward documenting Jinja functions remain static in YAML files on RPC until modified #2330 as a known limitation / unsupported use of invocation_id + run_started_at

There's one more thing I'd like to do around logging when we turn on partial parsing by default: #3886