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>

syslog.go

@@ -38,6 +38,12 @@ type Entry struct {
 
 	SourceType string            // "syslog" or "docker"
 	Extra      map[string]string // additional fields (docker: container, image, …)
+
+	// Not stored. Wait: the producer can be slowed down when the queue is full
+	// (Docker, host logs) instead of losing the message. Done: called once the
+	// message is stored in VictoriaLogs or in the disk spool.
+	Wait bool
+	Done func()
 }
 
 // Record returns the entry in the shape stored in VictoriaLogs and returned by the API.