Add --event-buffer-size flag, recreate EVENT_HISTORY once configured

[jtcohen6]

Follow-on to #4411

• Add --event-buffer-size flag to main.py
• When GLOBAL_HISTORY is being created right now, flags have not yet been resolved, so it's just using the default value set in the flags module (10k). We need to recreate global EVENT_HISTORY once all configs are available.

The downside: We might lose 1-2 events that are fired before we call setup_event_logger(). Currently, this is just deprecation warnings for renamed configs in dbt_project.yml.

The problem: Currently, setup_event_logger is being called much later than it should be from main.py, because it needs to wait for log_path to be resolved (based on config in dbt_project.yml).

Flags are resolved earlier than that, so we should refactor this to either:

• Split setup_event_logger into different functions for setting up global EVENT_HISTORY + STDOUT_LOG vs. global FILE_LOG, since only the latter needs to know log_path
• Set up FILE_LOG to use RotatingFileHandler with delay + emit, i.e. buffer events until it knows where log_path is, then drop them in the file. This is the approach we used to take with the legacy logger.

Checklist

• I have signed the CLA
• 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
• I have updated the CHANGELOG.md and added information about my change