Fix usage message for LogCabin --config <file>, fix default server ID
Replace initlog.py with LogCabin --bootstrap

It's better to do this in C++:
 - There's currently no way to read the config file from Python, which
   leads to manually specifying things on the command line
 - Different storage backends need to be initialized differently, and
   it's not ideal to have that logic both inside C++ and in Python