implement topk.

This is _completely_ untested. It compiles. It will probably do
nothing else (well, besides crashing Bro).
well, a test that works..

Note: merging top-k data structures is not yet possible (and is
actually quite awkward/expensive). I will have to think about
how to do that for a bit...
Merge branch 'topic/seth/metrics-merge' into topic/bernhard/topk
Merge remote-tracking branch 'origin/topic/seth/metrics-merge' into topic/bernhard/topk
topk for sumstats
make the get function const
add serialization for topk
implement merging for top-k.

I am not (entirely) sure that this is mathematically correct, but
I am (more and more) getting the feeling that it... might be.

In any case - this was the last step and now it should work
in cluster settings.
make size of topk-list configureable when using sumstats
Merge remote-tracking branch 'origin/master' into topic/bernhard/topk
add topk cluster test
Merge remote-tracking branch 'origin/topic/seth/metrics-merge' into topic/bernhard/topk
fix warnings
fix memory leaks
in cluster settings, the resultvals can apparently
been uninitialized in some special cases
Merge remote-tracking branch 'origin/topic/bernhard/topk' into topic/bernhard/topk
add sum function that can be used to get the number of total
observed elements.

Add methods to merge with and without pruning (before only merge
method was with pruning, which invalidates the number of total
observed elements)
Merge remote-tracking branch 'origin/topic/seth/metrics-merge' into topic/bernhard/topk
Merge remote-tracking branch 'origin/topic/robin/metrics-merge' into topic/bernhard/topk
t merge Merge remote-tracking branch 'origin/master' into topic/bernhard/topk