Factoring mode

[rossica]

Implement a factoring mode for finding the largest prime factors for a given number.
The number would be specified by the --start parameter.
This necessitates a number of changes to the network protocol:

• Either RegisterClient or WorkItem needs to specify that this is a factoring job, not a simple search job. Also to include the number to test against.
• A new ReportFactoringWork message type which reports primes found that aren't factors, as well as the primes found which are factors. (this is under the assumption that clients are searching for primes, instead of being given a list of primes to test.)
  It would return a type-count-[size,value]+ for the discovered non-factors, and the discovered factor (if any.)

Alternatively, the factoring mode would accept a file with a bunch of discovered primes (that are smaller than sqrt(SP), where SP is the semiprime to factor), and would hand out chunks of those primes to clients to test against the number to factor. This requires the changes to RegisterClient or WorkItem as described above, but with the twist that the WorkItem message includes a list of primes to test against SP.