Start confd in notify mode if we can

This patch changes the default model—where confd is always started in
polling mode—to a model where if possible we enable inotify mode
before starting any of the background threads.

There are some tricky details here: if we enable inotify, we should
not modify the server state after wards, as inotify events could have
already fired and took ownership; therefore we presume that inotify
will work and set reloadModel=ReloadNotify, and only if we fail to
enable it we change it back to polling mode.

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