Remove thresholds from Keeper database. Change encryption mechanism for next public keys.

[SmithSamuelM]

Old next key digests in KERI key event messages were combined with XOR with the threshold serialized digest to produce a single digest derived next key list/threshold committment. In order to reproduce this combined digest the Keeper needed to store the next threshold along with the Next key pairs. This was changed with partial rotation where the next key list of digests is provided in its own field and the next key threshold in its own. Because the next threshold is now published in the KEL, there is no need to recover it from the keystore Keeper. The only thing that the Keystore needs to recover is the next key pairs.