packages: Update source url in rpm spec
common: Add custom exception

This commit introduces an exception which contains a message.
tests: Fix race in test beginning

Sometimes automatic tests were failing because writing data started before
every chunkserver has registered its space in the master server. This commit
introduces a binary which can check how many chunkservers are ready to work,
and makes a good use of it.
mount: Add a class which limits I/O

This class can be used to introduce an I/O limiting mechanism
in the mountpoint.