Fail write_file if the content will cause carbonadod to go over system capacity

As specified in the config.toml, check if the capacity specified will be exceeded when write_file is called. We can just check early on, the length of file_bytes, and do a worst-case heuristic of 2x on that size. Also keep in mind, that size is in bytes, but capacity is measured in gigabytes.

The size of only one volume needs to be checked, then it can be multiplied by FEC_M.