add threaded compilation

concerning and closing #465
description of $cur variable fixed
make splint happy
fix (some) race conditions for tests

running the testsuite in multithreaded mode breaks some tests,
especially if run after ```make cleanup```
shredder test thread safety
more thread safe sequniq tests
fix copy paste errors for shredder tests
use apt caching in travis
Merge pull request #467 from Garonenur/travis

add threaded compilation