broker: remove op deletes an actor

{"type":"remove","actor"} shuts the actor down normally, drops its
mailbox and supervisor, and deletes /actor/<id>/. Native actors cannot
be removed. Test in cli/tests/host.rs, documented in PROTOCOL.md.

PROTOCOL.md

@@ -109,7 +109,7 @@ Any request can be answered with `{"type": "fs_error", "errno": "no-entry"}`.
 
 Requests to `broker`. The answer is a message whose `reply_to` is the ID of
 the request: the request's `type` in past tense (`spawned`, `upgraded`,
-`rolled_back`, `stopped`, `started`, `subscribed`, `unsubscribed`,
+`rolled_back`, `stopped`, `started`, `removed`, `subscribed`, `unsubscribed`,
 `scanned`, `exiting`), or `{"type": "error", "reason": "..."}`.
 
 | `type` | Fields | Effect |
@@ -118,6 +118,7 @@ the request: the request's `type` in past tense (`spawned`, `upgraded`,
 | `upgrade` | `actor`, `wasm`, `source`?, `built_by`? | test, then swap the version |
 | `rollback` | `actor`, `version` | go back to an older version |
 | `stop`, `start` | `actor` | stop or start an actor |
+| `remove` | `actor` | stop the actor (normal shutdown), then delete its whole folder `/actor/<id>/` (all versions, `data/`, `public/`, logs); answer `{"type": "removed", "actor"}`. Messages to it are then undeliverable, and the ID can be spawned again. An unknown actor or a native one (`broker`, `console`, `cargo`, `fs`) is an `error` |
 | `subscribe`, `unsubscribe` | `actor` | receive (or not) what `actor` publishes |
 | `scan` | | boot new or changed `actor.wasm` files |
 | `exit` | | answer `exiting`, stop every actor, and end the `sausage` process (a wrapper like `nix run .#loop` starts it again, updated) |