Disk buffer for batches VictoriaLogs cannot take, lossless Docker positions, non-blocking reverse DNS

- Batches that fail go to /data/spool (SPOOL_MAX_MB, 1 GiB by default) and
  are sent again oldest first; retries no longer block the store loop and
  follow the shutdown context.
- Docker and host logs wait for room in a full queue instead of being
  dropped; the Docker position only moves once a line is stored or spooled.
- Reverse DNS no longer holds up the syslog listeners, with an LRU cache
  and a cap on concurrent lookups.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

histogram_test.go

@@ -153,7 +153,7 @@ func TestHistogramHandler(t *testing.T) {
 {"_time":"2000-01-01T00:00:00Z","severity":"info","hits":"9"}
 `, recent.Format(time.RFC3339), recent.Format(time.RFC3339), older.Format(time.RFC3339))
 	}}
-	store := NewStore("http://vl", 10, 10, time.Second)
+	store := NewStore("http://vl", 10, 10, time.Second, nil)
 	store.streamClient.Transport = vl
 	a := &API{store: store}
 
@@ -207,7 +207,7 @@ func TestHistogramDayInParis(t *testing.T) {
 {"_time":"2026-09-21T20:00:00Z","severity":"info","hits":"100"}
 `
 	}}
-	store := NewStore("http://vl", 10, 10, time.Second)
+	store := NewStore("http://vl", 10, 10, time.Second, nil)
 	store.streamClient.Transport = vl
 	a := &API{store: store}
 	from := time.Date(2026, 9, 10, 0, 0, 0, 0, time.UTC)