libhdfs++: Reimplement Status object as a normal struct

hdfs::Status is doing all sorts of reinterpret casts on a block of memory referenced by a char *. Using a char *, casting to a wider type, and dereferencing can cause fun alignment issues.

As far as I can tell that data layout in status can be boiled down to:

class Status {
  int code;
  std::string msg;
}

This avoids doing manual memcopies in the copy ctor and delete[]s in the dtor. It will also get rid of boilerplate null checks and casts.