Taker on_pubkey() should verify existence of CoinJoinTx object

[AdamISZ]

Here: https://github.com/chris-belcher/joinmarket/blob/master/lib/taker.py#L271-L272

The code assumes that when the pubkey message is received from the taker, the self.cjtx object already exists. This will not be the case if the CoinJoinTx.init() call has not completed. In the current version of the code, this won't happen, but it makes sense to put a sanity check like:

while not self.cjtx:
                      time.sleep(2)
                      debug("waiting for CoinJoinTX instantiation")

[AdamISZ]

It would also make sense to put self.cjtx = None before the call to CoinJoinTx() constructor in start_cj, so that there will not be a mistaken belief that the cjtx has been created (in the above code) when it hasn't (it's an old/stale one)