only use the first address of a storage key to invoke VP

[cwgoes]

This may be an issue w/the execution model.

[cwgoes]

We should remove the logic which parses all addresses for a key, and replace it with explicit inclusion in the verifiers list (and only parse the first address).

[cwgoes]

With this change:

• we will need to manually trigger other VPs in whatever VP should be triggering them (e.g. multitoken, proof-of-stake)
• we will need to rework vp_user to perform only signature validation, and move proof-of-stake etc. validation logic to the respective VPs

@tzemanovic will take this on; @batconjurer will review