Garbage collect only popular content (i.e. semi-pin)

[JesseWeinstein]

I think it would be neat to add an option to the garbage collector that caused it to avoid garbage collecting things with fewer than N providers (by doing a ipfs dht findprovs on each hash first). A sort of semi-pin.

[Kubuxu]

There is plan of much more complex GC: #130

DHT look-up operations are quite expensive but it would be nice feature to have.