demo: provide a command in 'cockroach demo' to persist the data

[knz]

While experimenting with cockroach demo it's possible that a sequence of interactive commands has placed the data in an "interesting state" that wants further investigation, but the precise list of steps to arrive there is unclear.
It would be desirable to be able to issue a simple command that makes the cockroach demo persistent.

This is currently already possible using the following (more complex) sequence of steps:

1. in the demo shell, use select address from crdb_internal.gossip_nodes to find the node address of the demo session
2. in a separate shell start a new node with a persistent store with --join and the address given at 1
3. decommission node 1 in this cluster that now has 2 nodes. This will move all the ranges from the demo session (in-memory store) to the persistent store.
4. stop the demo session. Now the data directory for node created in step 2 has a perfect copy of the demo session. This is sufficient to reuse the data because the demo replication factor is 1.

It would be nice if these steps could be automated somehow.

Jira issue: CRDB-6344

[knz]

cc @jordanlewis

[jordanlewis]

Cool idea!

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