Add label API to wallet RPC.
This is one step towards #3816 ("Remove bolt-on account system") although it doesn't
actually remove anything yet.
These initially mirror the account functions, with the following differences:
- These functions aren't DEPRECATED in the help
- Help mentions 'label' instead of accounts. In the language used, labels are
associated with addresses, instead of addresses associated with labels. (unlike
with accounts.)
- Labels have no balance
- No balances in `listlabels`
- `listlabels` has no minconf or watchonly argument
- Like in the GUI, labels can be set on any address, not just receiving addreses
- Unlike accounts, labels can be deleted.
Being unable to delete them is a common annoyance (see #1231).
Currently only by reassigning all addresses using `setlabel`, but an explicit
call `deletelabel` which assigns all address to the default label may make
sense.
Thanks to Pierre Rochard for test fixes.
This change only updates strings and adds RPC aliases, but should simplify the
implementation of address labels in
https://github.com/bitcoin/bitcoin/pull/7729, by getting renaming out of the
way and letting it focus on semantics.
The difference between accounts and labels is that labels apply only to
addresses, while accounts apply to both addresses and transactions
(transactions have "from" and "to" accounts). The code associating accounts
with transactions is clumsy and unreliable so we would like get rid of it.
