APT_DELIMITED_READ_SIZE environment variable in DataStage

By default, the DataStage reads ahead 500 bytes to get the next delimiter. For streaming inputs (socket, FIFO, and others) this behavior is suboptimal, since the DataStage might block (and not output any records). When DataStage reads a delimited record, it reads this many bytes (minimum legal value is 2) instead of 500. If a delimiter is NOT available within N bytes, N is incremented by a factor of 2 (when this environment variable is not set, this factor changes to 4).