Handle cached responses via deferred execution (experimental)
Handle cached responses via deferred execution (experimental)

As an alternate way of avoiding the forced 1s delay in WebKit browsers when
handling cached responses when in the background, defer execution of the handler
function using the `spf.async` package. For evaluation and testing, guard this
behavior behind a `experimental-defer-response-cache` config flag.

Progress on #337

scrapp-uk/spfjs,nicksay/spfjs,nicksay/spfjs,theanh/spfjs,nicksay/spfjs,tchiotludo/spfjs,nicksay/spfjs,scrapp-uk/spfjs,theanh/spfjs,theanh/spfjs,youtube/spfjs,youtube/spfjs,tchiotludo/spfjs,tchiotludo/spfjs,tchiotludo/spfjs,nicksay/spfjs,theanh/spfjs,scrapp-uk/spfjs,scrapp-uk/spfjs,youtube/spfjs,tchiotludo/spfjs,youtube/spfjs,scrapp-uk/spfjs,theanh/spfjs,youtube/spfjs