Remove dag parsing from airflow db init command

[SamWheating]

When running airflow db init, Airflow will parse all of the DAGs in the configured dag folder sequentially in a single process. When there are a large number of DAGs present this can significantly slow down the time it takes for the db init command to run.

In my opinion, initializing the DB and populating it with data are separate tasks and shouldn't be combined into a single function. The background DAG processor is also much faster at parsing files and populating the DB due to using multiprocessing.

I propose splitting the bootstrapping of the DagBag out into a separate function (so as to not introduce any changes to the test setup / teardown process) and removing it from the db init and db reset commands.

Additionally, removing this PR might be required for AIP-43 anyways

Let me know if there's anything I'm missing here, or if there's an explanation for parsing DAGs here which I may have missed.

[potiuk]

Nice one! @kaxil @ashb @ephraimbuddy -> WDYT?

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.