### Background

Currently, our CRDT uses three different types of timestamps:

| clock type | representation | purpose |
|-----|----------------|----------|
| `Local` | replica id + u32 | uniquely identifies operations |
| `Lamport` | replica id + u32 | provides a consistent total ordering
for all operations |
| `Global` | N local clocks | fully defines the partial ordering between
all concurrent operations |

All text operations include *each* type of timestamp. And every
`Fragment` in a buffer's fragment tree contains both a local and a
lamport timestamp.

### Change

An operation can be uniquely identified by its lamport timestamp, so we
don't really need a concept of a local timestamp. In this PR, I've
removed the concept of a local timestamp. Version vectors
(`clock::Global`) now store vectors of *lamport* timestamps.

Eliminating local timestamps reduces the memory footprint of a buffer by
four bytes per fragment, reduces the size of our `UpdateBuffer` RPC
messages, and reduces the amount of data we need to store in our
database for channel buffers. It also makes our CRDT a bit easier to
understand, IMO, because there is now only one scalar value that we
increment per replica.

It's possible I'm missing something here though. @as-cii, @nathansobo
it'd be good to get your 👀