perf: add batch packet processing with recvmmsg/sendmmsg

[datagutt]

Summary

• Use recvmmsg to receive up to 64 packets in a single syscall
• Use sendmmsg to broadcast to all connections in a single syscall

Changes

Receive batching (srtla_handler.cpp):

• New process_packets() function using recvmmsg to receive up to 64 packets at once
• Each packet processed via process_single_packet() helper

Send batching (srt_handler.cpp):

• Broadcast to all connections using sendmmsg instead of individual sendto calls
• Sends to all connections in a single syscall

Benefits

• Reduced syscall overhead under high packet rates
• Improved CPU efficiency for high-throughput scenarios
• Inspired by Moblin's batch receive pattern

Dependencies

This PR is based on #9 (broadcast control packets to all connections)

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch perf/batch-packet-processing

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