clock: Cleanup ReplicaId, Lamport and Global (#40600)

- Notable change is the use of a newtype for `ReplicaId`
- Fixes `WorktreeStore::create_remote_worktree` creating a remote
worktree with the local replica id, though this is not currently used
- Fixes observing the `Agent` (that is following the agent) causing
global clocks to allocate 65535 elements
- Shrinks the size of `Global` a bit. In a local or non-collab remote
session it won't ever allocate still.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/worktree/src/worktree.rs

@@ -656,7 +656,7 @@ impl Worktree {
 
     pub fn replica_id(&self) -> ReplicaId {
         match self {
-            Worktree::Local(_) => 0,
+            Worktree::Local(_) => ReplicaId::LOCAL,
             Worktree::Remote(worktree) => worktree.replica_id,
         }
     }