net: convert sockbuf waiting to use waitqueues

Add sockbuf::sb_cc_wq for using instead of msleep(&sb->sb_cc).  This paves
the way for lockless wakeups for net channels, as the wait primitive is now
thread::wait_for() instead of msleep().  In addition this reduces lock
acquisition and improves netperf bandwidth by about 10%.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>