add support for upstream groups

[tpetr]

• Adds optional group field to UpstreamInfo class (defaults to DEFAULT if absent)
• Adds additional upstreamGroups field to ServiceContext class (the handlebars templates use this). Upstreams without a group value are added to the default group.
• The original upstreams list in ServiceContext is not affected -- this will contain all upstreams.

@ssalinas

[ssalinas]

@tpetr meant to comment yesterday, two things:

• won't need a migration, since it's at the end of the upstream string and we use original path for deletions, should be fine
• We should probably still have the validation in there so the group value doesn't mess up our zk path. Why not do what we did for rackId and requestId where we default it to empty in the constructor? or could have an actual default there if that makes more sense

[tpetr]

Thanks for the feedback, I have an internal TODO to re-add validation on the group field.

[tpetr]

this is mostly just stylistic, but I'd personally make this a static method named something like UpstreamInfo.upstreamsAndGroupMatch(UpstreamInfo a, UpstreamInfo b)