wallet: Add GetPrefixCursor to DatabaseBatch

In order to get records beginning with a prefix, we will need a cursor
specifically for that prefix. So add a GetPrefixCursor function and
DatabaseCursor classes for dealing with those prefixes.

Tested on each supported db engine.

1) Write two different key->value elements to db.
2) Create a new prefix cursor and walk-through every returned element,
   verifying that it gets parsed properly.
3) Try to move the cursor outside the filtered range: expect failure
   and flag complete=true.

Co-Authored-By: Ryan Ofsky <ryan@ofsky.org>
Co-Authored-By: furszy <matiasfurszyfer@protonmail.com>

src/wallet/sqlite.h

@@ -15,12 +15,21 @@ struct bilingual_str;
 namespace wallet {
 class SQLiteDatabase;
 
+/** RAII class that provides a database cursor */
 class SQLiteCursor : public DatabaseCursor
 {
 public:
     sqlite3_stmt* m_cursor_stmt{nullptr};
+    // Copies of the prefix things for the prefix cursor.
+    // Prevents SQLite from accessing temp variables for the prefix things.
+    std::vector<std::byte> m_prefix_range_start;
+    std::vector<std::byte> m_prefix_range_end;
 
     explicit SQLiteCursor() {}
+    explicit SQLiteCursor(std::vector<std::byte> start_range, std::vector<std::byte> end_range)
+        : m_prefix_range_start(std::move(start_range)),
+        m_prefix_range_end(std::move(end_range))
+    {}
     ~SQLiteCursor() override;
 
     Status Next(DataStream& key, DataStream& value) override;
@@ -57,6 +66,7 @@ public:
     void Close() override;
 
     std::unique_ptr<DatabaseCursor> GetNewCursor() override;
+    std::unique_ptr<DatabaseCursor> GetNewPrefixCursor(Span<const std::byte> prefix) override;
     bool TxnBegin() override;
     bool TxnCommit() override;
     bool TxnAbort() override;