Daml: pass all memo key-value pairs via memo only

@mrdziuban : I've had a chat today with @moritzkiefer-da about our (my) design decision to allow setting keys both via memos and via metadata. We convinced ourselves that this is not a good idea for the following reasons:

1. It can be confusing.
2. Machines can easily build the memo, they don't need the metadata support.
3. In the worst case, this can be used for an attack.

What do you think about switching all lock operations to use memo-based parameter passing only? If you agree, could you do it against the locking/m1-daml PR?