bitbucketserver: Increase client rate limit to 8 req/s

Created by: tsenart

This commit increases each bitbucketserver.Client rate limit from 2 to 8 requests per second. The previous number didn't account for all the extra things we're now doing with the Bitbucket Server API, namely changesets and permissions syncing.

In the context of testing the Bitbucket Server Plugin, a large customer has reported that their Bitbucket Server instance receives ~100 req/s.

With those numbers in mind, I think it's OK for us to max out at 8 req/s by default. This is so that the permissions and changeset syncing takes a little less time, now that we're planning to ship background permissions syncing in 3.14.

@ryanslade is going to work on overhauling our rate limit story, including allowing admins to define themselves what rate limit they want, but for now I think we need this to make background permissions syncing usable.