Update collectd, the backbone of Luci statistics, to 5.5.1

Refresh patches. Main changes:
 - Remove 500-upstream-parallel-build-fix.patch (implemented upstream)
 - Add 600-fix-libmodbus-detection.patch to revert an upstream change
   that broke libmodbus detection in collectd's configure script

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

Update collectd, base of Luci statistics, to 5.5.0.

Patches have been refreshed.
write-graphite plugin is now enabled. (see #1351)

I have compile-tested all plugins with ar71xx,
and real-life tested the following plugins:
Conntrack, Processor (CPU), DNS, Interfaces, Wireless, System Load,
Memory, Ping, Uptime

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>