cmd/gitserver: Enable producer-consumer pipeline for async cloning

Created by: indradhanush

In this commit, we stop creating goroutines for each async clone request and instead enable the producer-consumer pipeline to manage async cloning of repos. This should help with avoiding goroutine spikes during rebalance operations in gitserver. We also remove the metrics that were introduced previously to aid us in verifying our approach works as intended.

And now that they have performed their duties exceptionally well, we can let them retire and add them to our hall of fame. They've earned their pensions.