Performance: Improve validation path and cached connection properties

[pglombardo]

Description

• Move most validations out of hot-path
• Use "null-coalescing assignment operator" (which is a real operator name)
• MQTT5PublishMessage: Move most validations out of publish hot code path
• Cached connection properties, fast path improvements & tests

Related Issue

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix

Checklist

• I've written tests (if applicable) for all new methods and classes that I created. (rake test)
• I've added documentation as necessary so users can easily use and understand this feature/fix.