[feature]: Multi-RFQ send

[GeorgeTsagk]

Description

The daemon currently grabs the first available asset channel when trying to send a payment, and will acquire a single RFQ for that asset channel alone. If that fails for any reason, the operation fails, without trying any other candidate channels or quotes.

We want to change that, so that when sending a payment multiple asset channels are considered, and multiple RFQs are acquired. The payment lifecycle from the PoV of tapd should also support a dynamic selection of asset channels and RFQs, which is adaptive to failures.

A brief summary of what needs to be done:

• Acquire quotes for all suitable asset channels, and consider all of them for sending the payment
• Payment lifecycle should take into account the multiple asset channels and quotes, and be able to adapt the next attempt when previous one fails

Investigation:

• Can we somehow re-use LND's sharding algorithm in order to not re-write something similar when fragmenting the asset amount over multiple channels/quotes?