[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, 1 insertions, 0 deletions

diff --git a/src/wallet/walletdb.h b/src/wallet/walletdb.h
index 040aa092e1..a73d727c0c 100644
--- a/src/wallet/walletdb.h
+++ b/src/wallet/walletdb.h
@@ -204,6 +204,7 @@ public:
     bool WriteAccountingEntry(const uint64_t nAccEntryNum, const CAccountingEntry& acentry);
     bool ReadAccount(const std::string& strAccount, CAccount& account);
     bool WriteAccount(const std::string& strAccount, const CAccount& account);
+    bool EraseAccount(const std::string& strAccount);
 
     /// Write destination data key,value tuple to database
     bool WriteDestData(const std::string &address, const std::string &key, const std::string &value);