Replace Block Height to Block Hash map

[fedekunze]

The HeaderHash getter and setter mapping from the store can be replaced by the staking module Keeper's HistoricalInfo that contains the header and the validator set.

Actionable Items:

• Add staking keeper
• Create StakingKeeper interface on the interfaces.go file, defining the expected functions:

GetHistoricalInfo(ctx sdk.Context, height int64) (types.HistoricalInfo, bool)

• Remove the header hash store functions, keys, etc