speed up scoped_loaded_collection in item search a bit

Reduces items#index.json by about 10ms per request on my box, development with
cache_classes=true. The time isn't huge, but is worthwhile.
Use item proxies for JSON caching

That is, once we get our list of IDs from the search engine, only
fetch records whose JSON we don't already have cached.

It's simpler here to use as_json, but it'd probably be even faster
if I figure out how to serve a plain JSON string from a Rails
controller. In the meantime, requests of entirely cached items
are coming in at about 85ms on average on my box (dev, cache
classes, many items), about 10ms better than the last
iteration.
use proxies for item html, too

Some lame benchmarking on my box, dev, cache classes, many items:

No proxies:
    Fresh JSON:  175,  90,  90,  93,  82, 88, 158, 150, 85, 167 = 117.8
    Cached JSON: (none)
    Fresh HTML:  371, 327, 355, 328, 322, 346 = 341.5
    Cached HTML: 173, 123, 175, 187, 171, 179 = 168

Proxies:
    Fresh JSON:  175, 183, 269, 219, 195, 178 = 203.17
    Cached JSON:  88,  70,  89, 162,  80,  77 = 94.3
    Fresh HTML:  494, 381, 350, 334, 451, 372 = 397
    Cached HTML: 176, 170, 104, 101, 111, 116 = 129.7

So, overhead is significant, but the gains when cached (and that should be
all the time, since we currently have 0 evictions) are definitely worth
it. Worth pushing, and probably putting some future effort into reducing
overhead.

On production (again, lame), items#index was consistently averaging
73-74ms when super healthy, and 82ms when pets#index was being louder
than usual. For reference is all. This will probably perform
significantly worse at first (in JSON, anyway, since HTML is already
mostly cached), so it might be worth briefly warming the cache after
pushing.