Use an inline cache when accessing Application.injector

Improves application performance by about 3–5% in some benchmarks by eliminating calls to Guice when making requests.

@jroper, you might be interested in this one.
Feedback welcome on this, but the current plan for the migration path is to have 2.4 as source compatible as possible. This means that the old global approach will still be supported and it should be a straightforward upgrade. The new DI approach will also be available and underlying the old APIs. Both approaches can be used in an application allowing a gradual adoption of the new APIs.

We're then thinking of a 2.5 release which is basically 2.4 with these objects and APIs all deprecated. The 2.5 release provides all the warnings to encourage users to migrate fully to the new DI way to do things. After 2.5 the global static APIs are removed.