Allow HDFS backup/restore buffer size to be configured

There's a default hardcoded buffer size setting of 4096 in the HDFS code which means in particular that restoring a backup from HDFS takes a long time. Copying multi-GB files from HDFS using a buffer as small as 4096 bytes is very inefficient. We changed this in our local build used in production to 256kB and saw a 10x speed improvement when restoring a backup. Attached patch simply makes this size configurable using a command line setting, much like several other buffer size values.