Merge bitcoin/bitcoin#26780: rpc: simplify scan blocks

b922f6b526 rpc: scanblocks, add "completed" flag to the result obj (furszy)
ce50acc54f rpc: scanblocks, do not traverse the whole chain block by block (furszy)

Pull request description:

  Coming from https://github.com/bitcoin/bitcoin/pull/23549#pullrequestreview-1105712566

  The current `scanblocks` flow walks-through every block in the active chain
  until hits the chain tip or processes 10k blocks, then calls `lookupFilterRange`
  function to obtain all filters from that particular range.

  This is only done to obtain the heights range to look up the block
  filters. Which is unneeded.

  As `scanblocks` only lookup block filters in the active chain, we can
  directly calculate the lookup range heights, by using the chain tip,
  without requiring to traverse the chain block by block.

ACKs for top commit:
  achow101:
    ACK b922f6b526
  TheCharlatan:
    ACK b922f6b526

Tree-SHA512: 0587e6d9cf87a59184adb2dbc26a4e2bce3a16233594c6c330f69feb49bf7dc63fdacf44fc20308e93441159ebc604c63eb7de19204d3e745a2ff16004892b45

test/functional/rpc_scanblocks.py

@@ -49,6 +49,7 @@ class ScanblocksTest(BitcoinTestFramework):
         assert blockhash in out['relevant_blocks']
         assert_equal(height, out['to_height'])
         assert_equal(0, out['from_height'])
+        assert_equal(True, out['completed'])
 
         # mine another block
         blockhash_new = self.generate(node, 1)[0]