twitter-server: add option to ResourceHandler to load resources from disk on every resource request

Problem

When iterating on resources (e.g. css, js) hosted by ResourceHandler, a dev must restart the server after each minor change to see its effect in the browser, which greatly increases iteration time.

Solution

Add a constructor parameter to ResourceHandler and some factory methods to give it more flexibility in how its resources are loaded, e.g., loading the resource from disk on every request.

Result

When developing locally on a properly-configured service, a developer can edit a resource (css/js), refresh the browser, and see the change immediately, without restarting the service.

RB_ID=538741
twitter-server: Clarify "Migration Guide" finagle-stats step

Problem

The twitter-server migration guide doesn't mention that you need
to add finagle-stats to your classpath.

RB_ID=545270