[collectd] Possible stability fixes for rrdcached plugin

[Toni Ylenius]

Sebastian Harl and Florian Forster,

I had stability problems with 5.0.0. I use rrdcached to store the values. When 
I added or removed new hosts or values collectd crached randomly. 

I merged all changes from collectd-5.0 branch and following sequential 
Sebastian's changes which I thought were important [1]

5caecf5ba53ad31fa0626591c1ccc91fb47af955
ccc69cc64e79170e09eb3eae9cb9d2d22c6f06e6
887bc2aa36a2b48ab28b0b0138bd55411efe55e6
e4517555fbb51d317ac4ca76dc8cfb3b8c1f5410
27769fc777da38494c3f8e2297af9099fbec0bc0
f2f52fa0700944578af732abf40fc40f3f65d0a6

and one earlier Florian's change so that previous changes applied

776e65a46fb7e69931267b9a4380ec56cb325386

As a result I got stable system. Thus I ask If you could merge the changes 
to 5.0 if you also believe that they fixed the stability problem.

I will report if I encounter any stability problems in the long run.

Thanks
Toni Ylenius

[1] A core dump had threshold related backtrace and so I searched threshold 
and rrchached related fixes from the master branch.
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