Resolve paths for views asynchronously

[aredridel]

This eliminates the only post-startup synchronous I/O in Express.

In addition, this makes the view.lookup method non-private, intended to be used by view engines to resolve paths to partials, and, if the View class is extended, could be used for other resources such as internationalized messages.

[dougwilson]

Nice! Anyway we can add to this a way to prevent stampeding (which the sync version protected from) to the default lookup implementation?

[aredridel]

Oh, yes! I think I could work that in.

[aredridel]

Updated.

[gabrielcsapo]

👍