Prevent empty sessions from being stored (lazy session creation)

[sdogruyol]

Summary

This PR addresses #52 by implementing lazy session creation. Sessions are now only persisted to storage when data is actually written to them, preventing empty sessions from accumulating.

Problem

Previously, when env.session was accessed, a new session was immediately created in storage even if no data was written to it. This caused issues with:

• Bots and web crawlers that don't retain cookies, creating many empty sessions
• Unnecessary storage usage and potential performance degradation over time

Solution

• Removed the create_session call from Session#initialize and Session#reset
• Sessions are now created lazily - the storage engines already support creating sessions on first write
• This is a minimal change that leverages existing engine behavior