[FIX] disallow concurrent access to the static PRNG

- otherwise the same random number might be produce twice which could
  lead to all kinds of apocalypses
- [FIX] (re)-named critical blocks (should be namespaced with OPENMS_)
- proposed solution to issue #956
Merge pull request #958 from hroest/fix/threadsafe_uniqueid

[FIX] disallow concurrent access to the static PRNG (fixes #956)