PrefetchCache: extract and re-use LRUCache from SnapshotCache

[seanpdoyle]

What

First, extract the LRUCache (named after the "least recently used"
caching policy) class from the SnapshotCache. The implementation
is largely unchanged, with the following exceptions:

1. accept toCacheKey argument to coerce key values
2. rename this.snapshot property to a more generic this.entries
3. rename snapshot arguments to entry
4. rename location arguments to key

Next, implement SnapshotCache in terms of the LRUCache. Since it
served as the original implementation, it is mostly an empty class that
passes the toCacheKey utility function to the constructor, and
provides a snapshots property alias that returns the this.entries
property.

Also implement the PrefetchCache as an extension of the LRUCache.
Rather than storing a reference to the "hot" entry, construct a cache of
size 1. When a new entry is put into the cache, the old entry is
expired. To supplement the "least recently used" purge strategy, also
declare a setInterval function to purge expired entries every
50ms.

Finally, change all PrefetchCache.get call sites to pass the URL
instance directly, rather than the String. Once passed as the
argument, the PrefetchCache.get implementation uses the existing
toCacheKey utility function already used by the SnapshotCache.
Rename any PrefetchCache.setLater call sites to instead invoke
PrefetchCache.putLater (named to match the SnapshotCache and
LRUCache "put" method).

Why

By adjusting the underlying PrefetchCache implementation to expand
upon an existing concept, it opens up possibilities for further
customization and configuration. Future changes could support
client-provided cache size and purge interval configuration to adjust
link prefetching behavior intervals to suit application-specific.

[seanpdoyle]

@brunoprietog I'm sorry to ping you again for review, but the original version felt like the wrong abstraction. Upon further reflection and consideration, I decided to try and re-purpose existing concepts (like the SnapshotCache class), rather than superficially refactoring the prior implementation.