Prune the list of supported consensus methods

We currently have 13 supported consensus methods.

In 0.3.3, it is likely that prop282 will add 1 more, and prop283 will add 2 more.

Maybe we should prune this list eventually, because it will let us simplify our code, and make votes smaller, less expensive to calculate, and reduce authority RAM requirements (due to fewer microdescs).

It has almost no impact on consensus size.

Here's how we could work out what to prune:

By mandatory feature:

We are currently locked into using consensus method 16 or later in the public network, because 0.2.9 and later require ntor keys, and 0.2.9 clients use microdescriptors by default.

We may add more mandatory features in 0.3.3 and 0.3.4.

By supported tor version:

On May 1, 2018, we will stop supporting 0.2.5, and only support 0.2.9 and later. This means that all supported non-alpha versions will support consensus methods 25 and later. (Or, if we count 0.2.9 alpha versions, it's 22 and later.)