Support multiple Put and Delete operations

[joshua-kim]

Currently we're restricted to a single PutCurrentValidator and DeleteCurrentValidator per block. This issue is to enable us to do an arbitrary amount per state diff.

One example is that if we:

1. Have a state diff that puts a validator
2. Add another diff on top of that that deletes, and then puts the validator (an update)

Diff (2) should be a net add for the validator. If we try to add another delete in diff (2) this will not work, because now the net change is a Add -> Delete, which is a No-op in the current implementation where we would have expected a net change of a delete.

[yacovm]

#5003