[wallet] [rpc] introduce 'label' API for wallet

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.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 170e60d485..99b09ec407 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -549,7 +549,7 @@ public:
 };
 
 /**
- * Internal transfers.
+ * DEPRECATED Internal transfers.
  * Database key is acentry<account><counter>.
  */
 class CAccountingEntry
@@ -989,6 +989,7 @@ public:
     std::map<CTxDestination, CAmount> GetAddressBalances();
 
     std::set<CTxDestination> GetLabelAddresses(const std::string& label) const;
+    void DeleteLabel(const std::string& label);
 
     isminetype IsMine(const CTxIn& txin) const;
     /**
@@ -1184,7 +1185,7 @@ public:
 
 
 /** 
- * Account information.
+ * DEPRECATED Account information.
  * Stored in wallet with key "acc"+string account name.
  */
 class CAccount