Add LRU cache for AA albums/tracks

The two caches provide information on tracks for `lookup()` calls,
reducing network IO and latency:

    flx@t420:~$ time mpc add gmusic:album:B6dlshnrjk4ucvcmyptc6p35pwi

    real	0m0.700s
    user	0m0.001s
    sys		0m0.001s

    flx@t420:~$ time mpc add gmusic:album:B6dlshnrjk4ucvcmyptc6p35pwi

    real	0m0.006s <-- dropped to 1/100 \o/
    user	0m0.001s
    sys		0m0.001s

Cache size is set to 1024 objects each.
It may make sense to increase size of the track cache.