Tx sync endpoint waits for sync to answer

[chedieck]

In the endpoint:
http://localhost:3000/address/transactions/[address]

... if [address] is unseen, we will first sync it fully so later the same endpoint can serve the txs from our db. Problem is: right now it is waiting for sync to finish to answer with {status:200, message: "Sync has started."}, which is obvisouly wrong.

So let's make so that it does start the sync, just as it already does, but answers immediately with that same response.