topology: wake the vacuum dispatcher when a worker frees quota (#11436)

* topology: wake the vacuum dispatcher when a worker frees quota

The dispatch loop slept a fixed 10s whenever every pending volume was
waiting for a per-server quota slot, so a sweep took volumes x 10s
regardless of how fast the compactions were. Workers now signal on a
buffered channel after crediting quota; the dispatcher waits on it with
the 10s sleep kept only as a timeout.

* master: add -vacuumIntervalSeconds to tune the automatic sweep interval

The 14-minute base interval was a literal inside the refresh loop while
every neighbouring vacuum knob was already a flag. Defaults to 840s,
unchanged.

* topology: keep the 14 minute floor on the vacuum interval

A zero-valued MasterOption or a negative -vacuumIntervalSeconds left the
sweep sleeping only its jitter, so treat non-positive intervals as the
previous default.

weed/command/server.go

@@ -98,6 +98,7 @@ func init() {
 	masterOptions.volumeSizeLimitMB = cmdServer.Flag.Uint("master.volumeSizeLimitMB", util.DefaultVolumeSizeLimitMB, "Master stops directing writes to oversized volumes.")
 	masterOptions.volumePreallocate = cmdServer.Flag.Bool("master.volumePreallocate", false, "Preallocate disk space for volumes.")
 	masterOptions.maxParallelVacuumPerServer = cmdServer.Flag.Int("master.maxParallelVacuumPerServer", 1, "maximum number of volumes to vacuum in parallel on one volume server")
+	masterOptions.vacuumIntervalSeconds = cmdServer.Flag.Int("master.vacuumIntervalSeconds", 840, "seconds between automatic vacuum sweeps")
 	masterOptions.defaultReplication = cmdServer.Flag.String("master.defaultReplication", "", "Default replication type if not specified.")
 	masterOptions.garbageThreshold = cmdServer.Flag.Float64("master.garbageThreshold", 0.3, "threshold to vacuum and reclaim spaces")
 	masterOptions.metricsAddress = cmdServer.Flag.String("master.metrics.address", "", "Prometheus gateway address")