Reduce goroutine creation when cloning repos

Created by: indradhanush

Currently when we receive requests to clone a new repo we spawn a goroutine and wait until we are allowed to clone based on the number of other concurrent clones happening.

https://github.com/sourcegraph/sourcegraph/blob/1c9e6f78cc6494318a5e3974975aa996b80e69dc/cmd/gitserver/server/server.go#L1280

During rebalance operations this leads to a huge spike in go routines which consumes a lot of memory and introduces the danger that the gitserver pod is OOM killed.

Instead, we should create n goroutines on startup, where n is the number of concurrent clones allowed and add clone requests to an internal queue