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

core/src/store/fst/backup.rs

@@ -20,6 +20,12 @@ impl FstStorePool {
         // Create backup directory (full path)
         fs::create_dir_all(path)?;
 
+        // NOTE: The FST store directory gets created when the first FST is
+        //   consolidated, so there is nothing to back up until then.
+        if !self.fst_store_config.path.exists() {
+            return Ok(());
+        }
+
         // Proceed dump action (backup)
         self.dump_action(
             "backup",