Fix large shares when small payload and large multiplicity

[akonring]

In many settings it may be advantageous to configure the VID scheme to use large multiplicity.
However, the current implementation of the ADVZ scheme will force polynomials to be of size multiplicity * recovery_threshold even for small payloads.
Instead, the disperse function should use the lowest value for multiplicity such that payload_len $$\leq$$ multiplicity * recovery_threshold.

[ggutoski]

There are lots of ways to solve this problem, but all of them are bad ergonomics. After some discussion we decided the least bad solution is what's suggested by OP: silently use the smallest multiplicity needed to fit the entire payload into a single polynomial. We'll rename multiplicity -> max_multiplicity and update docs to explain that this arg allows the caller to specify only the maximum multiplicity that can be used for dispersal.