'label' and 'account' APIs for wallet
-------------------------------------

A new 'label' API has been introduced for the wallet. This is intended as a
replacement for the deprecated 'account' API. The 'account' can continue to
be used in V0.17 by starting bitcoind with the '-deprecatedrpc=accounts'
argument, and will be fully removed in V0.18.

The label RPC methods mirror the account functionality, with the following functional differences:

- Labels can be set on any address, not just receiving addresses. This functionality was previously only available through the GUI.
- Labels can be deleted by reassigning all addresses using the `setlabel` RPC method.
- There isn't support for sending transactions _from_ a label, or for determining which label a transaction was sent from.
- Labels do not have a balance.

Here are the changes to RPC methods:

| Deprecated Method       | New Method            | Notes       |
| :---------------------- | :-------------------- | :-----------|
| `getaccount`            | `getaddressinfo`      | `getaddressinfo` returns a json object with address information instead of just the name of the account as a string. |
| `getaccountaddress`     | n/a                   | There is no replacement for `getaccountaddress` since labels do not have an associated receive address. |
| `getaddressesbyaccount` | `getaddressesbylabel` | `getaddressesbylabel` returns a json object with the addresses as keys, instead of a list of strings. |
| `getreceivedbyaccount`  | `getreceivedbylabel`  | _no change in behavior_ |
| `listaccounts`          | `listlabels`          | `listlabels` does not return a balance or accept `minconf` and `watchonly` arguments. |
| `listreceivedbyaccount` | `listreceivedbylabel` | Both methods return new `label` fields, along with `account` fields for backward compatibility. |
| `move`                  | n/a                   | _no replacement_ |
| `sendfrom`              | n/a                   | _no replacement_ |
| `setaccount`            | `setlabel`            | Both methods now: <ul><li>allow assigning labels to any address, instead of raising an error if the address is not receiving address.<li>delete the previous label associated with an address when the final address using that label is reassigned to a different label, instead of making an implicit `getaccountaddress` call to ensure the previous label still has a receiving address. |

| Changed Method         | Notes   |
| :--------------------- | :------ |
| `addmultisigaddress`   | Renamed `account` named parameter to `label`. Still accepts `account` for backward compatibility if running with '-deprecatedrpc=accounts'. |
| `getnewaddress`        | Renamed `account` named parameter to `label`. Still accepts `account` for backward compatibility. if running with '-deprecatedrpc=accounts' |
| `listunspent`          | Returns new `label` fields. `account` field will be returned for backward compatibility if running with '-deprecatedrpc=accounts' |
| `sendmany`             | The `account` named parameter has been renamed to `dummy`. If provided, the `dummy` parameter must be set to the empty string, unless running with the `-deprecatedrpc=accounts` argument (in which case functionality is unchanged). |
| `listtransactions`     | The `account` named parameter has been renamed to `dummy`. If provided, the `dummy` parameter must be set to the string `*`, unless running with the `-deprecatedrpc=accounts` argument (in which case functionality is unchanged). |
| `getbalance`           | `account`, `minconf` and `include_watchonly` parameters are deprecated, and can only be used if running with '-deprecatedrpc=accounts' |