Merge bitcoin/bitcoin#22263: refactor: wrap CCoinsViewCursor in unique_ptr

7ad414f4bf doc: add comment about CCoinsViewDBCursor constructor (James O'Beirne)
0f8a5a4dd5 move-only(ish): don't expose CCoinsViewDBCursor (James O'Beirne)
615c1adfb0 refactor: wrap CCoinsViewCursor in unique_ptr (James O'Beirne)

Pull request description:

  I tripped over this one for a few hours at the beginning of the week, so I've sort of got a personal vendetta against `CCoinsView::Cursor()` returning a raw pointer.

  Specifically in the case of CCoinsViewDB, if a raw cursor is allocated and not freed, a cryptic leveldb assertion failure occurs on CCoinsViewDB destruction (`Assertion 'dummy_versions_.next_ == &dummy_versions_' failed.`).

  This is a pretty simple change.

  Related to: https://github.com/bitcoin/bitcoin/issues/21766
  See also: https://github.com/google/leveldb/issues/142#issuecomment-414418135

ACKs for top commit:
  MarcoFalke:
    review ACK 7ad414f4bf 🔎
  jonatack:
    re-ACK 7ad414f4bf modulo suggestion
  ryanofsky:
    Code review ACK 7ad414f4bf. Two new commits look good and thanks for clarifying constructor comment

Tree-SHA512: 6471d03e2de674d84b1ea0d31e25f433d52aa1aa4996f7b4aab1bd02b6bc340b15e64cc8ea07bbefefa3b5da35384ca5400cc230434e787c30931b8574c672f9

src/rpc/blockchain.cpp

@@ -2391,7 +2391,7 @@ static RPCHelpMan scantxoutset()
             LOCK(cs_main);
             CChainState& active_chainstate = chainman.ActiveChainstate();
             active_chainstate.ForceFlushStateToDisk();
-            pcursor = std::unique_ptr<CCoinsViewCursor>(active_chainstate.CoinsDB().Cursor());
+            pcursor = active_chainstate.CoinsDB().Cursor();
             CHECK_NONFATAL(pcursor);
             tip = active_chainstate.m_chain.Tip();
             CHECK_NONFATAL(tip);
@@ -2590,7 +2590,7 @@ UniValue CreateUTXOSnapshot(NodeContext& node, CChainState& chainstate, CAutoFil
             throw JSONRPCError(RPC_INTERNAL_ERROR, "Unable to read UTXO set");
         }
 
-        pcursor = std::unique_ptr<CCoinsViewCursor>(chainstate.CoinsDB().Cursor());
+        pcursor = chainstate.CoinsDB().Cursor();
         tip = chainstate.m_blockman.LookupBlockIndex(stats.hashBlock);
         CHECK_NONFATAL(tip);
     }