macaroons: initial implementation of macaroon based accounting

[guggero]

In this PR we add a new accounting system to the macaroons package.
This first implementation only partially satisfies the features requested in #291 but further PRs will follow.

As we introduce it in this first PR, an account is a simple construct that has a balance and an optional expiration date. The balance represents satoshis that can be spent by the "owner" of the account.
Every invoice that is paid from/with an account reduces the balance of that account by the payment amount plus fees.
Once the balance of an account is 0, further payments will be refused from/with that account.

Applying an account to a macaroon is a restriction. The bearer of a macaroon that is locked to an account will only be able to spend at most as many satoshis as the account's balance.
If a macaroon is not locked to an account, there is no restriction.

Accounts only assert a maximum amount spendable. Having a certain account balance does not guarantee that the node has the channel liquidity to actually spend that amount.

There are three commands in lncli that can be used to manage accounts:

• createaccount balance [expiration_date] Creates a new account with the given balance and the optional expiration date.
• listaccounts Lists all accounts that are currently stored in the account
  database.
• removeaccount id Removes the account with the given ID.

Example output of listaccounts:

{
    "accounts": [
        {
            "id": "945ab38d3890c267",
            "initial_balance": "100",
            "current_balance": "100",
            "last_update": "1546254113",
            "expiration_date": "0"
        }
    ]
}

A new macaroon that is locked to an account can then be created using the delegatemacaroon command:
lncli --macaroonpath /some/dir/payment.macaroon delegatemacaroon --save_to /some/dir/account.macaroon --account_id abcdef01abcdef01
Of course this only makes sense if we can create a macaroon that is only allowed to pay invoices, once #1160 is merged.

With this first version there are several limitations that will be addressed in further PRs:

• Accounts cannot be replenished.
• Only payment methods (SendPayment, SendPaymentSync, SendToRoute and SendToRouteSync) are currently checked for the account constraint. It can be discussed if the ListAccounts method should only show the current account when the macaroon is locked to an account.
• There is no link between accounts and the payments that have been paid from/with them.
• There are no accounts with on-chain balance.
• There are no accounts with a periodic balance (e.g. for subscription payments).

Depends on #1147 and #1152 (the first three commits of this PR will be removed as soon as these two PRs are merged to master).

[guggero]

Realized this probably shouldn't be built into lnd itself but some kind of wrapper library. Will continue working on this in a different context.