Link level bidirectional strict-forwarding

[GeorgeTsagk]

Description

A node that creates an invoice with the intention of receiving asset type X will want to settle invoice only if the HTLCs paying to it carry units of asset type X.

In our last iteration we added a guard to the AuxInvoiceManager which prohibits asset invoices being settled by HTLCs carrying pure btc (see related PR, and specific code section)

Problem

While this is partially effective, we still have a few open vectors:

• A btc invoice may still be paid for with HTLCs carrying assets
• An asset invoice may not be settled by btc HTLCs, but it may be settled by HTLCs carrying assets of a different type

Proposed Solution

• Add a guard against the 2nd direction violation (btc invoice <- asset HTLCs) (closed by Strict forwarding pt.2 #1244)
• Assert invoice-RFQ asset ID and HTLC asset ID match #1255

[dstadulis]

Closing as strict forwarding was delivered which was necessary for v0.5