routing+invoices: implement AMP secret sharing and combination

[Roasbeef]

This issue is related to lightning/bolts#658. Completion of this issue entails creating a new secretshare package (naming suggestion) within lnd that's able to:

• Statically create a set of N shares for apriori splitting use cases.
• Dynamically create a set of N shares (incrementally or adaptively) fo more adhoc splitting use case.
• Combine the relevant shares and perform key derivation to produce preimages and payment hashes related to an ultimate payment

Once the above is done (the libraries at the edges), we'll also need to then integrate this new combination logic into the invoice registry. Along the way we'll likely opt to create some sort of new HTLC combination+settling (deciding when it's possible to combine and also how to derive the pre-images) within the invoice registry.

I've left out most of the details w.r.t the additive secret sharing scheme as those are elaborated upon in detail within the linked issue. One other thing I've left out of this is the implication on the RPC interface, for calls such as ListPayments and ListInvoices.

[cfromknecht]

Closed by #4162