check for memory leaks

[maxklenk]

• [x] manual check

  I had a look at the code to search for memory leaks and didn't find any.

• [x] big tests

  I created a lot of large tables (1000 tables, 800000 rows each) to check if the memory consumption increases, but everything looks fine

• [ ] use http://valgrind.org/

  I was not able to compile Valgrind, can somebody please try it.