Slashing module must handle initial blocks differently

[haasted]

Describe the bug

The slashing module jails validators based on a ratio of missed to signed blocks due to the dynamic blocktime of the chain.

This creates a special case in the initial life of a chain where missing a low number of blocks results in a very large "missed ratio".

How to reproduce

Spin up a testnet and let one of the validators miss the first block. This will be interpreted as a "100% miss rate" and the validator will be immediately jailed.

Expected behavior

Allow the validators to miss a reasonable amount of blocks without getting slashed.

One way to solve this is to not jail a validator if blockCount in the slashing module's keeper is less than e.g. 100.

[haasted]

The fix applied in this issue is not sufficient.

The first block's blocktime will be from genesis.json and may thus be outside the signedBlocksWindow.