Improve proxy performance

[phillebaba]

Describe the problem to be solved

The proxy component of Spegel will copy data between response and request it has made. This copying when done with Go is done in user space, meaning the data has to be copied. The process can be slow and is a lot slower than doing this as a zero copy function. The bandwidth can be measured to be noticeably smaller when making a request through the proxy compared to directly to another Spegel instance.

Proposed solution to the problem

No response

Relates to #551 and #530