Decouple market dht implementation

abstract transport.dht.iterativeStore(transport, ...) out into transport.store(...).
Before saying anything else, let me first thank you for your enthusiasm in the cleanup of our codebase! It is much appreciated. Now, on with the review:
 
First of all, there is no reason to `return` anything, since the return value of `iterativeStore` is not used anywhere in the code. Just remove `return`.

Second, you have probably ommitted the actual function you meant to call:
```
self.dht.iterativeStore(...
```

Third, rememeber you are in Python, so the `self` argument is indirectly provided to the function called. You need to remove it from the arguments, otherwise the code will malfunction.

Fourth, as it is, `store` is just an intermediary to `dht.iterativeStore`. The entire code can be rewritten like this:
```
def store(self, *args, **kwargs):
    """
    Store or republish data.

    Refer to the dht module for further details.
    """
    self.dht.iterativeStore(*args, **kwargs)
```
The refactoring is much more self-evident now.

Please fix it and I will happily test and merge it. Sorry for my harsh style; we are a few days before feature-freeze for Beta 2.0 and I am quite overwhelmed with code that needs review.