DatasetReader holds back a complete row on a live stream #171

On a stream that stays open (a pipe, a socket, a file still being appended to), DatasetReader can hold a complete row until more bytes arrive or the stream ends. protowire-go returns the row as soon as its ) has been read. Go pins this in TestDatasetReader_CompleteRowIsNotHeldBack (encoding/pxf/dataset_scan_linear_test.go, #149 there): it writes a header and one row into a pipe one byte at a time, leaves the pipe open, and requires Next to return within 10 s for 16 row shapes.

Two causes, both from reading the code (not measured with a pipe)

1. The read blocks for a full chunk. DatasetReader::Pull calls std::istream::read(buf, 4096), which returns only once it has 4096 bytes or the input has ended. Go's io.Reader.Read returns whatever is there. So on a live stream, cpp waits for 4096 more bytes, or the end, even when the buffer already holds a complete row.
2. The scan's lookahead (PR pxf: DatasetReader ends a table only at the end of input or a directive (#164) #167). cpp's lexer has no sawEnd, so DatasetReader::Complete takes a token as complete only once four bytes follow it or the source has ended. A row whose ) is the last byte read waits for four more bytes. Go's lexer knows a one-byte token such as ) is complete. The fast path (TryWholeRow) covers a row that is already whole when Next starts. The scan path, for a row that arrived across reads, does not.

On input that ends, neither changes a result, only when the row is returned. The 590-case sweep in #167 matches Go.

Done when

• Pull returns what the stream has without waiting for a full chunk, e.g. readsome, or rdbuf()->sgetn after in_avail, with a blocking read of at least one byte only when nothing is buffered. Which of these is a design choice to make in the PR.
• A token whose end the lexer did not need to look past is complete at once. At least the one-byte punctuation tokens are, and ideally the lexer gets Go's sawEnd.
• A test in the shape of Go's: one row per case, fed byte by byte through a stream that stays open, returned by Next without the stream ending.