Excessive goroutines in recording proxy

[russjones]

Problem

The recording proxy uses 10k goroutines for the same load as a Teleport node which uses 1k goroutines during testing. This is due to the keepAliveLoop in lib/forward/sshserver.go.

This function ticks every 5 minutes and waits for 3 ticks before closing. Instead this function should return as soon as the server is closed. The tick period can also be reduced from 5 minutes to maybe 1 minute.