Improve upload throughput on fast connections

[got3nks]

Summary

The upload throttler uses hardcoded constants designed for ADSL-era connections (~128 KiB/s) that create artificial bottlenecks on modern fast connections. This patch dynamically scales parameters based on the configured upload rate while preserving identical behavior for slow connections (< 256 KiB/s).

• Scale minFragSize/doubleSendSize dynamically with data rate for fast connections, removing the fixed 1300/2600 byte caps that limited per-slot data per throttler pass
• Scale bandwidth banking tolerance to ~20ms worth of bandwidth, enabling efficient burst transmission instead of the fixed (slots * 512 + 1) byte cap
• Increase per-client payload buffer from 100 KiB to 8 MiB to prevent socket starvation at high per-slot speeds
• Increase packet chunk size from 10 KiB to 128 KiB in both standard and compressed packet paths, reducing per-packet header overhead
• Set TCP send/receive buffers to 512 KiB after successful connect, allowing the kernel to batch more data per read/write

Backward compatibility

All changes are fully backward compatible:

• Connections below 256 KiB/s use the original constants unchanged
• The two-pass fairness algorithm is preserved
• No wire protocol changes — works with all stock eMule/aMule clients
• Larger packets (128 KiB) are well within MAX_PACKET_SIZE (2 MB)
• Socket buffer sizes are local OS settings invisible to remote peers

Test results

Tested on a 1 Gbps dedicated server running aMule in Docker:

• Before: ~450 KB/s per upload slot (stock aMule)
• After: up to 8 MB/s per upload slot (18x improvement)
• Aggregate upload throughput with 25+ concurrent slots: ~12-22 MB/s
• Memory usage: 130-180 MiB under load (fluctuates with number of active upload slots)

Note: peak results were achieved in combination with #436 (uint16 → uint32 speed limits), which removes the 524 Mbps configuration cap. This PR alone still provides significant improvement on connections up to that limit.

Recommended configuration for fast connections

When using MaxUpload=0 (unlimited), aMule dynamically calculates the number of upload slots based on the current upload rate. This can result in a slow ramp-up where only a few slots are active initially. For best results on fast connections, set an explicit MaxUpload value rather than relying on unlimited mode:

• Stock aMule: MaxUpload=65534 (~524 Mbps, maximum before uint16 unlimited threshold)
• With Widen speed limit types from uint16 to uint32 #436 applied: MaxUpload=100000 or higher for gigabit+ connections

This ensures the full number of upload slots are available immediately.

Files changed

File	Changes
src/UploadBandwidthThrottler.cpp	Fragment size scaling + bandwidth banking
src/UploadClient.cpp	Per-client buffer + packet chunk size
src/LibSocketAsio.cpp	TCP send/receive buffers after connect

[got3nks]

Superseded by #444 — a cleaner, minimal version with just the two most impactful changes (adaptive payload buffer + adaptive chunk size), no throttler or socket layer modifications.