Merge #19325: wallet: Refactor BerkeleyDatabase to introduce DatabaseBatch abstract class

b82f0ca4d5 walletdb: Add MakeBatch function to BerkeleyDatabase and use it (Andrew Chow)
eac9200814 walletdb: Refactor DatabaseBatch abstract class from BerkeleyBatch (Andrew Chow)

Pull request description:

  In order to support alternative database systems, we need to have a generic `Batch` class. This PR adds a `DatabaseBatch` abstract class which is implemented by `BerkeleyBatch`. `DatabaseBatch` is now the class that is used by `WalletBatch` to interact with the database. To be able to get the correct type of `DatabaseBatch`, `BerkeleyDatabase` now has a `MakeBatch` function which returns a newly constructed `std::unique_ptr<DatabaseBatch>`. For `BerkeleyDatabase`, that will be `std::unique_ptr<BerkeleyBatch>`.

  The `Read`, `Write`, `Erase`, and `Exists` template functions are moved from `BerkeleyBatch`.

  Part of #18971

  Requires #19308 and #19324

ACKs for top commit:
  Sjors:
    re-utACK b82f0ca4d5
  MarcoFalke:
    ACK b82f0ca4d5 🌘
  meshcollider:
    LGTM, utACK b82f0ca4d5

Tree-SHA512: 6d2d41631c0983391dbecd702e881c6775b155c90b275df97f7157e42608ed251744f9d7ce5173d02a6c5cc38d90b611880fac7fa635d3d8c4d590681f56ac6a

src/wallet/bdb.cpp

@@ -841,3 +841,8 @@ bool BerkeleyBatch::HasKey(CDataStream&& key)
     int ret = pdb->exists(activeTxn, datKey, 0);
     return ret == 0;
 }
+
+std::unique_ptr<BerkeleyBatch> BerkeleyDatabase::MakeBatch(const char* mode, bool flush_on_close)
+{
+    return MakeUnique<BerkeleyBatch>(*this, mode, flush_on_close);
+}