Ignore/log errors while fetching servers for groups

This means that if there's an error such as a tenant being deleted (which happened in production), we'll still be able to collect metrics.

This is based on the pure-server-info-2 branch, so to look at the diff check out https://github.com/rackerlabs/otter/compare/pure-server-info-2...ignore-metrics-errors
parallel doesn't have concurrency limiting yet, that's why we have to return a list of Effects which allows the caller to perform() them all individually in a DeferredSemaphore. Note that this is only a doc change in this branch to reflect the already existing behavior -- it was incorrect before.