luci-app-statistics: Add backup/restore for RRD statistics

luci-app-statistics: Add backup/restore for RRD statistics

Add a backup/restore capability for rrd data storage in
luci_statistics.  The data storage is typically in /tmp and does not
survive reboot or sysupgrade.  This adds an option for the
administrator to configure the RRD plugin, so that the RRD data are
are preserved with a backup copy in the overlay file system.

This works for shutdown/reboot, sysupgrade (backup config files,
restore config files, and true sysupgrade).

Also fix a bug where starting luci_statistics for the first time would
not get a restart a running collectd: during install of the package
when it is not included in the base flashed image, collectd might be
started when it got installed/configured before this package gets
installed/configured.  So we need to check if it's running, and
restart it to use the luci_statistics configuration.

Signed-off-by: John Kohl <jtk.git@bostonpog.org>