Split whitepaper into layers more clearly

[michielbdejong]

• Sections 1, 2 and 3.3 of the whitepaper describe cycle resolution
• Section 3.1 describes my thoughts about bilateral ledgers from 2016, but this has since evolved (see SNAP)
• Section 3.1 also introduces the UNICURN approach which I moved aways from in favour of netting-at-equilibrium.
• Section 3.2 introduces bidirectional breadth-first search for loop detection

It could be better to make Decentralised Loop Detection (DLD) a separate thing (maybe an IETF ID or an IL-RFC, although routing seems to have been removed from scope for Interledger), and then in an updated LedgerLoops whitepaper refer to SNAP and DLD, and leave UNICURN/netting-at-equilibrium out of scope?