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/test/util.cpp

@@ -92,6 +92,17 @@ CTxDestination getNewDestination(CWallet& w, OutputType output_type)
     return *Assert(w.GetNewDestination(output_type, ""));
 }
 
+// BytePrefix compares equality with other byte spans that begin with the same prefix.
+struct BytePrefix { Span<const std::byte> prefix; };
+bool operator<(BytePrefix a, Span<const std::byte> b) { return a.prefix < b.subspan(0, std::min(a.prefix.size(), b.size())); }
+bool operator<(Span<const std::byte> a, BytePrefix b) { return a.subspan(0, std::min(a.size(), b.prefix.size())) < b.prefix; }
+
+MockableCursor::MockableCursor(const MockableData& records, bool pass, Span<const std::byte> prefix)
+{
+    m_pass = pass;
+    std::tie(m_cursor, m_cursor_end) = records.equal_range(BytePrefix{prefix});
+}
+
 DatabaseCursor::Status MockableCursor::Next(DataStream& key, DataStream& value)
 {
     if (!m_pass) {
@@ -175,7 +186,7 @@ bool MockableBatch::ErasePrefix(Span<const std::byte> prefix)
     return true;
 }
 
-std::unique_ptr<WalletDatabase> CreateMockableWalletDatabase(std::map<SerializeData, SerializeData> records)
+std::unique_ptr<WalletDatabase> CreateMockableWalletDatabase(MockableData records)
 {
     return std::make_unique<MockableDatabase>(records);
 }