Add support for client caching via Etags

[atuttle]

• Should only happen for GET requests
• Should be automatic; just enable it in settings
• Use a hash of the serialized result (only for textual responses) as the etag value

[atuttle]

Java has a method .hashCode() on all objects which should be perfect for our needs here -- it gives us a unique(ish?) identifier for anything (string/number/bool/struct/array/query) without having to first serialize the data.

hash(serializeJson(data)) vs data.hashCode()

While the result is not the same, the concept is: It's a unique identifier that will be the same whenever the data is the same. In theory, .hashCode() should be much faster, too.

Unfortunately, Railo does not currently implement this method correctly (or so I'm told).

Short term, I'm just going to use it (this is only in the 1.3-dev branch for now, after all) and hope that Railo fixes their implementation. Long term, if they choose not to change their implementation or don't seem like they'll get around to it any time soon, I'll have to find a way to detect that the code is running on Railo and use hash(serializeJson()) instead. It will be slower, but would work.

[atuttle]

The Railo guys have fixed .hashCode() - not sure what release it'll be in but we should document that. Got a question in to them to verify the correct minimum version.

[atuttle]

For reference, the Railo bug report.

[atuttle]

Documented: https://github.com/atuttle/Taffy/wiki/List-of-all-variables.framework-settings