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.
master: Update maxtotalspace on CS registration

Master keeps a variable indicating the highest amount of disk space
available on any chunkserver.

By omission, this variable wasn't updated during registration with
version 5x packets. Fix it.