Add store functions to the ChannelKeeper V2 that use the new provable paths

[chatton]

The ChannelKeeper V2 should include new functions which fetch and write to the store using the new paths outlined in the spec.

e.g. the equivalent of channelkeeperv1.SetPacketCommitment

ref cosmos/ibc#1148

[chatton]

Leaving issue in progress pending final spec changes

[DimitrisJim]

think @AdityaSripal mentioned these are very unlikely to change. Should we close or wait until cosmos/ibc#1144 is merged?

[chatton]

Linked issue has been merged, we should be good to close!