walletdb: Add MakeBatch function to BerkeleyDatabase and use it

Instead of having WalletBatch construct the BerkeleyBatch, have
BerkeleyDatabase do it and return a std::unique_ptr<BerkeleyBatch>

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);
+}