Decouple market dht implementation

abstract transport.dht.iterativeStore(transport, ...) out into transport.store(...).
Hi Renelvon,

Thank you for your review!

re left out iterativeStore: argl, yes… 

I added the arguments directly in store(), because I’m thinking of doing a proof-on-concept second transport over freenet, and for that a clearly defined API which does not have to change with dht implementation details would help.

But let’s go with this simpler version, given that the DHT likely still is in flux.