Add a cache for pending deals on API

[turinglabsorg]

@irenegia found following bug:

• She made a deal on the UI
• Deal didn't show up
• Provider delayed the confirm because it did a deposit first

So we have to make sure that:

• First of all we create an api endpoint to store directly the draft (protect the endpoint from abuse adding like 1 cached tx per address)
• Then api will update the cached tx if transaction is confirmed (it can be also delayed a bit on ETH mainnet, depending on gas used)
• This cached version must shows as "pending tx" on the UI
• Delay a little bit, like 5s, the redirect from "new deal" to "manage deals"

[turinglabsorg]

Since yesterday we changed the approach, from a security perspective allow anyone post data to the api is not secure. So we'll use this approach:

• We're caching transaction id inside deal record in database
• Client will start polling deals endpoint until last txid is found
• In any case we'll delay a bit the redirection between "new deal" and "manage deals"

[turinglabsorg]

Closing because this issue is now fixed.