[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.

test/functional/wallet_listreceivedby.py

@@ -140,7 +140,7 @@ class ReceivedByTest(BitcoinTestFramework):
         assert_equal(balance, balance_by_label + Decimal("0.1"))
 
         # Create a new label named "mynewlabel" that has a 0 balance
-        self.nodes[1].getlabeladdress("mynewlabel")
+        self.nodes[1].getlabeladdress(label="mynewlabel", force=True)
         received_by_label_json = [r for r in self.nodes[1].listreceivedbylabel(0, True) if r["label"] == "mynewlabel"][0]
 
         # Test includeempty of listreceivedbylabel