cooker/server: Fix up 100% CPU usage at idle

cooker/server: Fix up 100% CPU usage at idle

The recent inotify changes are causing a 100% cpu usage issue in the
idle handlers. To avoid this, we update the idle functions to optionally
report a float value which is the delay before the function needs to be
called again. 1 second is fine for the inotify handler, in reality its
more like 0.1s due to the default idle function sleep.

This reverts performance regressions of 1.5 minutes on a kernel build
and ~5-6 minutes on a image from scratch.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>