Configuration and environment variable improvements

[pwojcikdev]

This cleans up code related to overriding node configuration:

• Extracts environment variable reading, which is now done in a more generic, C++ way.
• Always prints info when an environment variable overrides config.
• Ensures that configured thread counts are in reasonable ranges. (And slightly tweaks default for request_aggregator)
• Converts is_<sanitizer> calls to constevals
• Adds ability to configure number of IO threads by environment variable

[pwojcikdev]

This also fixes a problem where default message processing threads would be 0, that's causing current develop to fail on macos runners.