Fix pavement.py bug, fix read_string bug...

 - Fix `pavement.py bug`:
  - Prior to this commit, the `generate_command_process_header` function
    was calling `_generate_protobuf_definitions`.  However,
    `generate_command_process_header` should call
    `_generate_command_process_header`.
 - Fix `read_string` bug:
  - Since, as of 474663b, `read_string` interprets the call-back
    argument as a `UInt8Array`, `sizeof(buffer.data)` no longer returns
    the length of the buffer.  Therefore, the code has been modified in
    this commit to extract the length of the array through the `length`
    attribute of the `UInt8Array` instance.