Add separate options for --input_prefix_length and --output_prefix_length

[salsferrazza]

LengthDelimitedOutputManager takes the --prefix_length command line option to size the int representing the prefix length for a message sent to the output stream. However, LengthDelimitedInputSource also uses the --prefix_length option for reading input streams that are length-prefixed.

The two options are distinct and should be separately instrumented. The clash namely occurs in a relatively rare situation, for example you are reframing messages and either narrowing or widening the prefix length as part of the process.