Update databases/rrdtool to 1.6.0
Most notably, this version of rrdtools.so is thread safe, so the
rrdtools_th.so library is removed.
Bump portrevision for depending ports due to shlib version bump.
For full changelog: http://oss.oetiker.ch/rrdtool/pub/CHANGES
Reviewed by: kwm
Differential Revision: D6168