rewrite DisconnectedBlockTransactions as a list + map

And encapsulate underlying data structures to avoid misuse.
It's better to use stdlib instead of boost when we can achieve the same thing.

Behavior change: the number returned by DynamicMemoryUsage for the same
transactions is higher (due to change in usage or more accurate
accounting), which effectively decreases the maximum amount of
transactions kept for resubmission in a reorg.

Co-authored-by: Cory Fields <cory-nospam-@coryfields.com>

src/validation.h

@@ -51,7 +51,7 @@ class CBlockTreeDB;
 class CTxMemPool;
 class ChainstateManager;
 struct ChainTxData;
-struct DisconnectedBlockTransactions;
+class DisconnectedBlockTransactions;
 struct PrecomputedTransactionData;
 struct LockPoints;
 struct AssumeutxoData;