Implement equivocations

[tomaka]

We should report when a block author authors two different blocks with the same slot.

Similarly, GrandPa should detect voting for two different blocks of the same height.

Very long term issue.

[tomaka]

We should report when a block author authors two different blocks with the same slot.

After chatting a bit, this should be done even if one of the blocks is not a descendant of the latest finalized block.
This complicates a bit the implementation.

[tomaka]

Another thing to note is that we need to keep in memory the votes that were downloaded when we perform a warp syncing, in order to make sure to detect double-voting with the warp syncing as well.

[tomaka]

Substrate source code for Babe equivocations: https://github.com/paritytech/substrate/blob/dddfed3d9260cf03244f15ba3db4edf9af7467e9/client/consensus/babe/src/lib.rs#L1094