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>

Some collectd plugins launch third-party plugins that have variation in
initialisation time (like libpcap). Recently (since kernel bump to 4.1)
the DNS plugin has been causing collectd to crash semi-randomly at startup
on MIPS based WNDR3700.

Debugging led to realisation that the DNS plugin seems to require at least
0.1s time to start, before the first data reading attempt starts.

By default, the first data read cycle starts immediately, while apaprently
some of the plugins may still be asyncronously initialising. To make things
safe, this patch adds 1 second delay before the first data read cycle.

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