tests: Fix descriptor leak test to work with valgrind

The test tried to obtain chunkserver pid using 'pidof mfschunkserver',
but this doesn't work when chunkserver is being run under valgrind.

Moreover, the default timeout multiplier of 5 turned out to be too low.

Use 'mfschunkserver test' instead of pidof and increase the timeout
multiplier to 10 to fix the above issues.
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
utils: Add file-overwrite

This commit adds a binary 'file-overwrite' which changes the content of given
file to be validated by file-validate.
It also does cosmetic changes in file-generate.

Closes: #128
master: Add chunk state update when cs disk marked as 'todel'

Two errors were found when chunkserver has been reloaded after marking disks
for removal:
* no changes in state of chunks;
* no changes in statistics.
They caused that the disk hasn't been prepared for removal. This commit fixes
this condition.