Make multiplexer constants configurable via env vars

[bdraco]

The following environment variables are available to override internal defaults:

• MULTIPLEXER_INCOMING_QUEUE_MAX_BYTES_CHANNEL - The maximum number of bytes allowed in the incoming queue for each multiplexer channel.
• MULTIPLEXER_INCOMING_QUEUE_LOW_WATERMARK - The low watermark threshold, in bytes, for the incoming queue for each multiplexer channel.
• MULTIPLEXER_INCOMING_QUEUE_HIGH_WATERMARK - The high watermark threshold, in bytes, for the incoming queue for each multiplexer channel.
• MULTIPLEXER_OUTGOING_QUEUE_MAX_BYTES_CHANNEL - The maximum number of bytes allowed in the outgoing queue for the multiplexer channel.
• MULTIPLEXER_OUTGOING_QUEUE_LOW_WATERMARK - The low watermark threshold, in bytes, for the outgoing queue for each multiplexer channel.
• MULTIPLEXER_OUTGOING_QUEUE_HIGH_WATERMARK - The high watermark threshold, in bytes, for the outgoing queue for each multiplexer channel.

[codspeed-hq]

CodSpeed Performance Report

Merging #390 will not alter performance

_{Comparing bdraco:multiplex_const_env_vars (4b5ffe1) with main (4df7a7f)}

Summary

✅ 2 untouched benchmarks

[ludeeus]

Thanks!