Decrease a relay buffer size for MIPS devices

[9seconds]

This is a logical continuation of @dolonet work on reducing memory overhead per connection. Since we have started to support MIPS hardware, it makes sense to have some optimizations in this regard.

Usually those devices are consumer hardware that runs either RouterOS or different flavors of OpenWRT. These devices are very limited in resources, so none gonna run large scale there. In this case it makes sense to decrease memory usage there by trading CPU cycles.

I haven't measured an impact because do not have such device available at this moment but here is how Opus measured a connection requirements:

Component	Size
Relay buffers (2×16KB)	~33 KB
Goroutine stacks (2×)	~33 KB
Doppel write buffer	~16 KB
TLS state (bufio + readBuf)	~8 KB
Doppel conn state	~4 KB
Other (cipher, ctx, wrappers)	~1 KB
Total	~95 KB

So, if we cut relay buffer sizes from 16 to 8, this gonna make ~17% difference.

[Toki2Me]

в целом интересно, я везде использует микроты) можно на слабенькие версии роутеров резервные прокси повесить, посмотреть на сколько это жизнеспособно