Add thread safeness

[matthiasbeyer]

This is the main issue for the thread safeness, which is (by now) totally missing.

Sub-issues for more concrete parts of this issue will be opened by @neithernut (if he agrees).

[matthiasbeyer]

@neithernut and I discussed and came to the conclusion that thread-safeness on the hashtable layer should be quite enough. If someone wants to add avl-node level threadsafeness, I'd welcome PRs on this (which have to include carefully done benchmarking), but we won't add it.

Therefor, I remove the type labels except the hashtable one.