Add test case and release notes for auto finalization

Summary:
As per title.

Depends on D2102

Test Plan:
  make check
  ./test/functional/test_runner.py abc-finalize-block

Reviewers: #bitcoin_abc, schancel, jasonbcox

Reviewed By: #bitcoin_abc, schancel, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2106

doc/release-notes.md

@@ -3,3 +3,7 @@ Bitcoin ABC version 0.18.5 is now available from:
   <https://download.bitcoinabc.org/0.18.5/>
 
 This release includes the following features and fixes:
+ - Add the finalized block concept. Finalized blocks cannot be reorged, which protects the network against deep reorgs.
+ - Add the `-maxreorgdepth` configuration to configure at what depth block are considered final. Default is 10. Use -1 to disable.
+ - Introduce `finalizeblock` RPC to finalize a block at the will of the node operator.
+ - Introduce a penalty to alternative chains based on the depth of the fork. This makes it harder for an attacker to do mid size reorg.

test/functional/abc-finalize-block.py

@@ -42,7 +42,7 @@ def run_test(self):
         alt_node.generate(10)
 
         # Wait for node 0 to invalidate the chain.
-        def wait_for_invalid_block(block):
+        def wait_for_invalid_block(node, block):
             def check_block():
                 for tip in node.getchaintips():
                     if tip["hash"] == block:
@@ -51,7 +51,13 @@ def check_block():
                 return False
             wait_until(check_block)
 
-        wait_for_invalid_block(alt_node.getbestblockhash())
+        # node do not accept alt_node's chain due to tip being finalized,
+        # even though it is longer.
+        wait_for_invalid_block(node, alt_node.getbestblockhash())
+        assert_equal(node.getbestblockhash(), tip)
+
+        # alt_node has mined 10 block, so tip must be invalid due to finalization.
+        wait_for_invalid_block(alt_node, tip)
 
         self.log.info("Test that an invalid block cannot be finalized...")
         assert_raises_rpc_error(-20, RPC_FINALIZE_INVALID_BLOCK_ERROR,