Random exceptions from data subsystem

[bingmann]

Sometimes (at least with zipnode) it crashes with

30: terminate called after throwing an instance of 'std::runtime_error'
30: [host 0 worker 1 000000] Worker 1 threw St13runtime_error
30: [host 0 worker 1 000001]   what(): object 1 not in repository @ /home/travis/build/thrill/thrill/thrill/data/repository.hpp:85
30:   what():  object 1 not in repository @ /home/travis/build/thrill/thrill/thrill/data/repository.hpp:85
30/42 Test #30: api_zip_node_test ...................***Exception: Other  2.06 sec

[bingmann]

this was fixed and due to a missing mutex lock