Change the polling thread sleep behaviour

Currently, the polling thread sleeps at the end of its body; the
original reason was that when the thread initially starts, we want to
load the config immediately (in the old-style config load model).

However, now we explicitly load the config, so we can make this thread
behave like the watcher thread: sleep first, then do the work. This
model improves for example the startup of the thread, by not redoing a
config check immediately after switching to polling mode.

We also increase the max idle poll rounds interval from 2 to 3; the
new behaviour means that we revert from polling to inotify after
around one second of no changes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>