The original libflow was written for f3write only, and
this was reflected in the name of fields and variables
throughout the code. This patch renames these fields and
variables to reflect the fact that libflow will eventually
support f3write AND f3read.
With really fast drives (i.e. >1GB/s), f3write needs to
write multiple files before a measurement can be made.
This patch enhances the flow library to deal with such fast drives.
