Improve client IP resolution for Stream logs

[kasparsd]

Fixes #1456.

• Rely on $_SERVER['REMOTE_ADDR'] as the canonical source for the request origin IP.
• Show a notice in admin if $_SERVER['REMOTE_ADDR'] is not specified.
• ~Fallback to $_SERVER['HTTP_X_FORWARDED_FOR'] only if $_SERVER['REMOTE_ADDR'] not present.~

Checklist

• [x] Project documentation has been updated to reflect the changes in this pull request, if applicable.
• [x] I have tested the changes in the local development environment (see contributing.md).
• [x] I have added phpunit tests.

Release Changelog

• Fix: Prefer $_SERVER['REMOTE_ADDR'] for the source of user IP and fallback to $_SERVER['HTTP_X_FORWARDED_FOR'] only in case REMOTE_ADDR is not available. Introduce a wp_stream_client_ip_address filter to adjust the trusted client IP.

[kasparsd]

@schlessera I feel like attempting the fallback behaviour might not be in our best interest here because it would actually be different from the previous behaviour. Let's make it a breaking change instead (does this warrant a 4.0.0 release?) and encourage everyone to use the newly introduced filter for adjusting the client IP address on environments where either REMOTE_ADDR is reporting wrong.

[calvinalkan]

@schlessera I feel like attempting the fallback behaviour might not be in our best interest here because it would actually be different from the previous behaviour. Let's make it a breaking change instead (does this warrant a 4.0.0 release?) and encourage everyone to use the newly introduced filter for adjusting the client IP address on environments where either REMOTE_ADDR is reporting wrong.

My 2C.

1. The fallback behaviour is insecure. People wil shoot themselves in the foot.
2. If PHP can't trust it's server to set REMOTE_ADDR correctly, or at all, all hope is lost. This should not be a consideration.
3. Yes, breaking change, 4.0.0