HPCC-9157 - Dfu server partition calculation not efficient when more than 2M
file sprayed

This improvement impact the partition calculation speed if any prefix defined
in dfuplus spray command.

The original version inserted the prefix headers into the existing partition
data. With large amount of data it takes long time (322 sec for 1M,
1445 sec for 2M and 9739 sec for 4M files).

The new approach merges original partition items with prefix headers into a
new array. At the end it copies the merged items into the original partition
array. (It takes 17 sec for 2M and 90 sec for 4M files).

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>