Technically, this helper function creates a random polynomial where the `root` parameter is a root of the polynomial, so `make_random_polynomial_with_root` is a better name.

There was no function for the refresh case, and since it's very similar to the recovery case, we define here a common method for both (`prepare_share_updates_with_root`)

The concept of "threshold" doesn't belong to the abstraction level of polynomials, which actually deal with degrees. For the record, it's always the case that `threshold == degree + 1`.

Test using secret reconstruction directly, rather than via ciphertext decryption.

These functions assumed access to an update polynomial, but we're actually using several distributed polynomials (one for each participant) and we only have access to their evaluated points.

Note that the commits before did the preparation to remove this now.

Updates for shares were not applied correctly (see diff in  L418-423). For the moment, we're recovering the same domain point since it facilitates debugging, but in the next commit I'll restore the test so there's a new validator, with a random domain point.

* Removed benchmark for random polynomial with root (not much value as it's essentially benchmarking arkworks)
* Commented recovery & refresh benchmarks for when #162 is solved