[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.

src/wallet/walletdb.cpp

@@ -161,6 +161,11 @@ bool WalletBatch::WriteAccount(const std::string& strAccount, const CAccount& ac
     return WriteIC(std::make_pair(std::string("acc"), strAccount), account);
 }
 
+bool WalletBatch::EraseAccount(const std::string& strAccount)
+{
+    return EraseIC(std::make_pair(std::string("acc"), strAccount));
+}
+
 bool WalletBatch::WriteAccountingEntry(const uint64_t nAccEntryNum, const CAccountingEntry& acentry)
 {
     return WriteIC(std::make_pair(std::string("acentry"), std::make_pair(acentry.strAccount, nAccEntryNum)), acentry);