Distinguish preconnect links as SPF-specific.

Instead parsing of `<link rel="preconnect">` in order to preresolve DNS for the
target and preconnect the socket, parse `<link rel="spf-preconnect">`.  This
makes it clear that the feature is SPF-specific.

Progress on #124.
Merge pull request #155 from nicksay/rename-link-connect

Distinguish preconnect links as SPF-specific.
Clear Performance Resource Timings before each navigate.

Ensure that Resource Timings are not incorrectly retrieved for previous
requests in Chrome (where the API is asynchronous) and that the buffer
size limit is not reached.  Clearing is done only before sending new
navigate requests to avoid accidentally removing unrelated Resource Timings
during a prefetch or load request.

Closes #115.