Store tip of the BTC light client as separate key in db

[KonradStaniec]

Currently retrieving tip of BTC Light client is implemented as retrieving the first element of reverese iterator of all the headers:
https://github.com/babylonlabs-io/babylon/blob/v0.15.0/x/btclightclient/keeper/state.go#L122

This proves to be inefficient. Given that retrieving the tip is frequent operation it worth investigating whether storing tip as separate entry improves the sitution.