Separate aux data generation and key refresh protocols

[fjarri]

While the question in #98 remains open, we can certainly implement a light version of it: separate the aux data generation and the key refresh, so that the user could choose how often to perform them independently. Aux data generation needs to generate primes, so it's relatively slow, but perhaps it can be done less often than key refresh.

[fjarri]

AuxGen was separated in #119, to be used in the threshold scenario. We cannot use CGGMP's KeyRefresh for it, and will probably have to adapt KeyResharing, so providing a separate KeyRefresh is not necessary for now.