Make light client contract upgradable

[gjermundgaraba]

Make light client contracts upgradable in the same way the other IBC contracts are.

[srdtrk]

I want to implement light client upgrabilitity through migrations in ICSCore. I think we don't need to use a proxy contract here as light clients which upgrade have no need to preserve their past state (actually, it is more secure to wipe past state).

I was thinking of using AccessControl contracts in openzeppelin to allow per-client migration in ICSCore. I'll proceed with this if we agree.