Fix memory leaks in HLS sessions, WriteBuffer, and RTSP UDP handling

[matteius]

• Add MaxSessions limit (100) to prevent HLS session accumulation
• Add session cleanup goroutine with 30-second monitoring interval
• Add MaxBufferSize (16MB) limit to HLS session buffers
• Add MaxWriteBufferSize (8MB) limit to WriteBuffer
• Add sync.Pool for UDP read buffers in RTSP conn to reduce allocations
• Add debug logging for session creation/expiration

These fixes address memory growth issues observed during extended operation where go2rtc memory would grow from ~150MB to 7GB+ over 2.5 days with usage from lightNVR.