SourceLayout: shuffle CommIO into DiskThreads library

SourceLayout: shuffle CommIO into DiskThreads library

While investigating the Windows port problems it became clear that the
CommIO object is not actually related to the rest of comm systems. It is
instead a dedicated higher level disk I/O pipe manager for DiskThreads.

Given that it causes build errors in comm.cc on Windows and does not
even need to be there this patch shuffles it into the DiskThreads
library. The build issues on Windows still exist but are now limited to
just that threads library and can be avoided temporarily with simple
./configure options.