Lnd: Add support for accounting-based macaroons

[aakselrod]

There is a large set of functionality that can be added by extending lnd's macaroon implementation by adding internal state. This partially draws on #286, but also adds root key deletion/expiration with garbage collection. This functionality could enable use cases described in issue #20 such as:

• Giving a process or user a macaroon with a predetermined, one-time budget
• Giving a process or user a macaroon with a predetermined, periodic budget
• Giving a process or user the ability to manually open channels and send on-chain payments
• Allowing a process or user to use only channels it has opened

This would be a pretty massive functionality addition as well as reimagining of much of the API. To do this, we would need to do the following:

• [ ] Add per-macaroon off-chain and on-chain balance tracking to the macaroon database with expiration and expired macaroon deletion.
• [ ] Add on-chain balance macaroon generation to NewAddress, NewWitnessAddress, and CloseChannel and consumption to OpenChannel, OpenChannelSync, SendCoins, and SendMany.
• [ ] Add off-chain balance macaroon generation to OpenChannel and OpenChannelSync and consumption to SendPayment, SendPaymentSync, and CloseChannel.
• [ ] Add RPCs for CheckMacaroonBalance, SubdivideMacaroonBalance, and CombineMacaroonBalance to manage macaroon balances.

There are other possibilities and potential issues to watch out for listed in my previous comment on issue #20, so this is only preliminary. This implementation is likely to get fairly complicated.

[guggero]

I'll start working on accounting-based macaroons. This issue lists features for several PRs IMHO, so I'll start with a basic off-chain balance macaroon. Then we'll see where we get from there.

[Kixunil]

I'm definitely interested in this. It would be useful for mitigating attacks against Joule browser extension.