[Feature Request]: double-sign detection

[tzdybal]

Implementation ideas

Currently during sync, blocks are processed in First Come, First Served manner. After first block at given height is processed, other at this height are completely ignored. In case of double-sign of blocks at given height no action is taken (as latter block is ignored).

This is a missed opportunity to detect double-sign of a block.

Currently sequencer is centralized, so double-sign attack should have very significant consequences and probably should be resolved via human consensus.

[tzdybal]

CC: @robert-zaremba