Use a worker pool to add concurrency limit on gif renders

[robbles]

This uses https://github.com/alitto/pond to add a middleware that wraps all gif rendering endpoints with a worker pool that limits concurrent executions to the number of CPUs, and allows up to 40 requests to sit in a queue until a worker frees up. After that limit is reached, requests with get a HTTP 429 instead to avoid hitting the overall Cloud Run active request limit.

This isn't exactly heavily tested, but I have poked at it locally a fair bit with lower numbers for the pool, and it seems to work as expected...? The worst case I'd expect is that this causes a memory leak if I got something wrong. I don't think it'll crash exporter, so it'll probably be a net improvement over intermittently denying all requests.

I also got a warning from running go mod tidy after adding the new dependency. It seems to happen on main as well, so I don't think I introduced it, but if I chose the wrong thing it might be making unnecessary changes to the deps:

github.com/BattlesnakeOfficial/exporter/http imports
	github.com/sirupsen/logrus imports
	golang.org/x/sys/unix loaded from golang.org/x/[email protected],
	but go 1.16 would select v0.5.0
github.com/BattlesnakeOfficial/exporter/http imports
	github.com/sirupsen/logrus imports
	golang.org/x/sys/windows loaded from golang.org/x/[email protected],
	but go 1.16 would select v0.5.0
github.com/BattlesnakeOfficial/exporter/http imports
	github.com/sirupsen/logrus imports
	golang.org/x/sys/unix imports
	golang.org/x/sys/internal/unsafeheader loaded from golang.org/x/[email protected],
	but go 1.16 would select v0.5.0

To upgrade to the versions selected by go 1.16:
	go mod tidy -go=1.16 && go mod tidy -go=1.17
If reproducibility with go 1.16 is not needed:
	go mod tidy -compat=1.17
For other options, see:
	https://golang.org/doc/modules/pruning