Usability improvements for z_importkey

- Add height parameter to z_importkey to reduce rescan range
- Change semantics of rescan parameter, so users can explicitly force a rescan
  for existing keys.

Closes #2032

qa/rpc-tests/wallet_1941.py

@@ -31,6 +31,14 @@ class Wallet1941RegressionTest (BitcoinTestFramework):
         connect_nodes_bi(self.nodes,0,1)
         self.sync_all()
 
+    def restart_second_node(self, extra_args=[]):
+        self.nodes[1].stop()
+        bitcoind_processes[1].wait()
+        self.nodes[1] = start_node(1, self.options.tmpdir, extra_args=['-regtestprotectcoinbase','-debug=zrpc'] + extra_args)
+        self.nodes[1].setmocktime(starttime + 9000)
+        connect_nodes_bi(self.nodes, 0, 1)
+        self.sync_all()
+
     def wait_and_assert_operationid_status(self, myopid, in_status='success', in_errormsg=None):
         print('waiting for async operation {}'.format(myopid))
         opids = []
@@ -93,8 +101,16 @@ class Wallet1941RegressionTest (BitcoinTestFramework):
         self.nodes[1].generate(101)
         self.sync_all()
 
-        # Import the key on node 1.
-        self.nodes[1].z_importkey(key)
+        # Import the key on node 1, only scanning the last few blocks.
+        self.nodes[1].z_importkey(key, 'true', self.nodes[1].getblockchaininfo()['blocks'] - 100)
+
+        # Confirm that the balance on node 1 is zero, as we have not
+        # rescanned over the older transactions
+        resp = self.nodes[1].z_getbalance(myzaddr)
+        assert_equal(Decimal(resp), 0)
+
+        # Re-import the key on node 1, scanning from before the transaction.
+        self.nodes[1].z_importkey(key, 'true', self.nodes[1].getblockchaininfo()['blocks'] - 110)
 
         # Confirm that the balance on node 1 is valid now (node 1 must
         # have rescanned)