cleanup
reduced number of malloc/frees during string template processing

We now re-use once allocated string template memory for as long
as the worker thread exists. This saves us from doing new memory
allocs (and their free counterpart) when the next message is
processed. The drawback is that the cache always is the size of
the so-far largest message processed. This is not considered a
problem, as in any case a single messages' memory footprint should
be far lower than that of a whole set of messages (especially on
busy servers).