storage,compute: unify timely clusters

[benesch]

Part of the multipurpose clusters epic (#17413).

As part of the initial cluster unification work (https://github.com/MaterializeInc/cloud/issues/4929), we unified storaged and computed into a single clusterd process, but left the storage and compute timely clusters separate. That is, a clusterd process hosts two timely clusters: one for storage and one for compute.

To support multipurpose clusters, we need to unify these two timely clusters. There are many design questions to shake out:

• Do we unify the command streams?
• If so, do we unify the command streams before we unify the timely clusters?
• What do we do with introspection relations like mz_compute_frontiers that will now contain compute and storage frontiers? See #10089 for a past pondering on the name of this relation.

cc @guswynn @aljoscha @antiguru @teskje

[guswynn]

More details in a braindump here: https://www.notion.so/materialize/ComputeState-StorageState-Unification-f8139794f9e449408ecd5244f4ae814b