lib: Refactore thread_process_fd

lib: Refactore thread_process_fd

thread_process_fd is looping over the read and write
fd's separately.  There is no need to do this individually.
loop over both the read and write fdset's at the same time.
This will improve select processing performance, especially
for large data sets.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>