add command to store/sync a given data size onto disk

[tomcucinotta]

in addition to the process, forward and reply commands, the distwalk should also support the possibility to specify a disk write operation that should be sync()ed on disk, before letting the next command be executed (e.g, reply or forward); similarly, we should have a disk read operation; for both, we should be able to specify the amount of data to write (alongside with the actual data to write, possibly) or read (causing that amount of data to be returned with a reply) as a parameter of the command.

[deRemo]

Working on this

[deRemo]

commit 287d3e5
commit 50dc783