wallet: implement BerkeleyROBatch

Implement ReadKey and HasKey of BerkeleyROBatch, and Next of BerkeleyROCursor.
Also adds the containers for records to BerkeleyRODatabase so that
BerkeleyROBatch will be able to access the records.

src/wallet/test/util.cpp

@@ -93,11 +93,6 @@ 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;