Add Default Implementations for DirectService

[davidbarsky]

This might make the task described in #136 easier. Namely, this addresses some of the “complexity” for services that don't need to spawn long-running futures, such as Buffer, certain kinds of middleware, and leaf services.

Note: this will need to be rebased atop of #149 to build successfully.

[davidbarsky]

Closing because:

• There's no mechanism of taking back a default in a library without a breaking change.
• Helper functions provide the same or greater degree of utility as default implementations while avoiding a large number of rakes.