Add rate limiting

Tried to add the [RateLimiter](https://github.com/brave-intl/bat-go/blob/master/middleware/rate_limiter.go#L15) in the initial implementation but it turns out it does not fit sync-server use case and we would be throttled when doing normal sync flow, so it was removed.
We should come back on this topic and add rate limiting either in ALB or in our go-sync server application.

Start by reading middleware/rate_limiter.go in the linked bat-go implementation and the go-sync server's sync flow. Determine whether the work belongs in the ALB or the Go application, and define rate limits that do not throttle normal synchronization. Done means rate limiting is implemented in the selected layer without disrupting normal sync.

Written by the indexing model from the issue text.