Previously, when loading Hypothesis into a page, the client would make
one search request for each and every URI found on the page, including
those found by the document metadata scanners (in the Annotator
"Document" and "PDF" plugins.)

This is wasteful of network bandwidth, and moreover should not be
necessary in most cases due to the ability to expand to equivalent URIs
on the server side.

The component responsible for coordinating communication across page
frames is the "crossframe" service. This services maintains a registry
of page frames (previously called "providers") and their associated
metadata. This commit reduces the metadata stored to simply the page
URL, or in the case of PDF documents, the fingerprint URN.

It's worth noting that this *does* change behaviour in one important
way: if a page contains document equivalence data that we've never seen
before, and one of the alternate URIs has already been annotated, then
this change will mean we do not load annotations until that page itself
has been annotated (and thus the document equivalence data updated). I
am assuming that this is an extreme edge case.