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.
