Collapse per-service Contexts into a single Context key.

[danjacques]

luci/gae is a single package that describes all available services, yet each service has its own set of Context keys. This results in several (10+) values being added to Context just for a simple installation of a full luci/gae service set.

One thought is to collapse those into a single struct that is shared between all of the services and gets copied and derived each time a service mutation is applied. This has several benefits:

• Services don't have to look each other up internally. This happens quite a bit when using info service from datastore, memcache, taskqueue, etc.
• Context is shallower, meaning that a dump will show one luci/gae entry, making it more useful in general.
• Overhead of Context lookups is reduced to one lookup per service call.