tests: Add complex test with valgrind

There was only a simple write and read test under valgrind. This commit
introduces a test which checks more use cases:
* mkdir
* create a file
* rename a file
* ls -l on dir
* truncate up & down some file
* set trash time
* make a snapshot
* copy-on-write
* concurrent writes and reads
* remove a file
* restart the master server
* restart a chunkserver

Closes #126