A set of fixes and features for wit\2

[drcpu-github]

The first four commits are part of https://github.com/witnet/witnet-rust/pull/2469.

The latter four commits are all new self-contained patches that introduce a fix or feature, but they build upon each other, so I kept them in the same branch.

[drcpu-github]

Pertaining to above discussions, this is what is currently implemented in the branch up to commit 9ec8d09:

1. All rf stakers are eligible to produce a block
2. Block winner comparison is power, vrf, block hash, in that order
3. If none of the eligible stakers produce a block, they will get slashed proportional to their rank
4. Validation code to make sure no one stakes on a validator with a different withdrawer address
5. A way to switch block times moving from V1_8 to V2_0
6. A bunch of small features and bugfixes

[drcpu-github]

Things to (potentially) still implement (of the top of my head):

1. Multiplication for the rf factor when more than one block is missing
2. Move wit/2 constants to a central file
3. Better ratios for slashing validators

[drcpu-github]

For the sake of an easier review, the block time patch is on https://github.com/witnet/witnet-rust/pull/2473.