fix(core): Make TRIGGER backup work while stores are in use

`TRIGGER backup` answered `ERR internal_error` in two common cases:

- a KV store was open in the pool (e.g. right after a `PUSH`): the backup
  opened the database a second time, which RocksDB refuses as its `LOCK`
  file is held. The pooled store is now backed up instead, after a flush.
- the KV or FST store directory did not exist yet (nothing ingested, or
  nothing consolidated yet): listing it failed. There is nothing to back up
  in that case.

Also enable the `trigger_backup` test, which covers both cases.

Fixes #274

server/tests/control_trigger.rs

@@ -41,9 +41,52 @@ fn trigger_consolidate() {
 }
 
 #[test]
-#[ignore = "Not supported by sonic_client yet (FIXME)"]
 fn trigger_backup() {
-    todo!()
+    let ctx = start_empty(|command| {
+        command
+            .env("SONIC_STORE__FST__GRAPH__CONSOLIDATE_AFTER", "3600")
+            .env("SONIC_STORE__FST__POOL__INACTIVE_AFTER", "3700")
+            .env("SONIC_STORE__KV__DATABASE__FLUSH_AFTER", "3600")
+            .env("SONIC_STORE__KV__POOL__INACTIVE_AFTER", "3700")
+    });
+
+    let multiplexer = SonicMultiplexer::new().unwrap();
+
+    let ingest =
+        SonicChannelIngestBlocking::connect(ctx.addr, "SecretPassword", &multiplexer).unwrap();
+    let control =
+        SonicChannelControlBlocking::connect(ctx.addr, "SecretPassword", &multiplexer).unwrap();
+
+    let backup_path = std::path::Path::new(env!("CARGO_TARGET_TMPDIR"))
+        .join("test-backups")
+        .join(ctx.id.to_string());
+    if backup_path.exists() {
+        std::fs::remove_dir_all(&backup_path).unwrap();
+    }
+
+    // NOTE: Nothing was pushed yet, so the store directories do not exist.
+    () = control
+        .trigger_backup(backup_path.to_str().unwrap())
+        .unwrap();
+
+    // NOTE: Pushing opens the KV store, which then stays open in the pool
+    //   while the backup runs, and leaves the FST changes pending.
+    ingest
+        .push("collection", "bucket", "object", "foo bar")
+        .unwrap();
+
+    () = control
+        .trigger_backup(backup_path.to_str().unwrap())
+        .unwrap();
+
+    let kv_backups = std::fs::read_dir(backup_path.join("kv"))
+        .unwrap()
+        .map(|entry| entry.unwrap().path())
+        .collect::<Vec<_>>();
+    assert_eq!(kv_backups.len(), 1);
+    assert!(std::fs::read_dir(&kv_backups[0]).unwrap().next().is_some());
+
+    std::fs::remove_dir_all(&backup_path).unwrap();
 }
 
 #[test]