Add --quorum command line argument (RIPD-563)

The `--quorum` command line argument accepts an integer and can be used to temporarily change the `validation_quorum` value without requiring editing of the configuration file. The value persists until execution is terminated.

Negative values are not allowed and using them will generate an error. An upper bound is not placed and any positive value will be accepted (up to `numeric_limits<int>::max()`). One open question is whether a value of `0` should be allowed.

@JoelKatz