Automatic control value num in buffers

[jackylee-ch]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
In WSCG, currently we use batchSize to control the values num in buffers. However, if we deal with large strings, it may lead to buffer overflows as failed to append string value to buffer.

Describe alternatives you've considered
We can hold the exception when we failed to append value to buffer and stop early as buffer has already full.