Dynamically adjust buffer_size

[lanlou1554]

For stream and iterator, buffer_size is one thing we should consider. If it is too small, then we have to do more frequent I/Os, but if it is too big, then we cannot serve too many requests at the same time due to fixed total memory size. Maybe we can adjust the buffer_size dynamically.

[lanlou1554]

#51 partially solves this issue. But maybe we can also consider the remaining space of our memory