walletdb: scope bdb::EraseRecords under a single db txn

so we erase all the records atomically or abort the entire
procedure.

and, at the same time, we can share the same db txn context
for the db cursor and the erase functionality.

extra note from the Db.cursor doc:
"If transaction protection is enabled, cursors must be
opened and closed within the context of a transaction"

thus why added a `CloseCursor` call before calling to
`TxnAbort/TxnCommit`.

Author: furszy

src/wallet/walletdb.cpp

@@ -1136,6 +1136,9 @@ bool WalletBatch::WriteWalletFlags(const uint64_t flags)
 
 bool WalletBatch::EraseRecords(const std::unordered_set<std::string>& types)
 {
+    // Begin db txn
+    if (!m_batch->TxnBegin()) return false;
+
     // Get cursor
     std::unique_ptr<DatabaseCursor> cursor = m_batch->GetNewCursor();
     if (!cursor)
@@ -1144,15 +1147,16 @@ bool WalletBatch::EraseRecords(const std::unordered_set<std::string>& types)
     }
 
     // Iterate the DB and look for any records that have the type prefixes
-    while (true)
-    {
+    while (true) {
         // Read next record
         DataStream key{};
         DataStream value{};
         DatabaseCursor::Status status = cursor->Next(key, value);
         if (status == DatabaseCursor::Status::DONE) {
             break;
         } else if (status == DatabaseCursor::Status::FAIL) {
+            cursor.reset(nullptr);
+            m_batch->TxnAbort(); // abort db txn
             return false;
         }
 
@@ -1163,10 +1167,16 @@ bool WalletBatch::EraseRecords(const std::unordered_set<std::string>& types)
         key >> type;
 
         if (types.count(type) > 0) {
-            m_batch->Erase(key_data);
+            if (!m_batch->Erase(key_data)) {
+                cursor.reset(nullptr);
+                m_batch->TxnAbort();
+                return false; // erase failed
+            }
         }
     }
-    return true;
+    // Finish db txn
+    cursor.reset(nullptr);
+    return m_batch->TxnCommit();
 }
 
 bool WalletBatch::TxnBegin()