Implement buffer growth multiplier

[rustyconover]

When dealing with very large redis keys such as 328mb strings
growing the buffer by 3 is very excessive. So if the request
is for larger than 256kb, only grow the buffer by that amount.

[BridgeAR]

@rustyconover thanks for bringing this up. I indeed did not consider such huge buffers. Setting the multiplier alone would not have helped in this case though.
The implementation I just pushed is actually a bit faster for big buffers than the former solution and works for the maximum possible typed array size (hiredis can't handle such big buffers btw), so this is a nice side effect 😄