vacuum: bound concurrent commits per volume server (#11658)

CommitCompact holds the volume's write lock for the whole commit, and
reads of that volume wait on it. Volumes compacted together tend to
finish together, so their commits land at once and their fsyncs
contend: each commit's lock hold grows with how many run alongside it,
even when the disk is far from its throughput limit.

Add -compactionMaxParallelCommits (volume.compactionMaxParallelCommits
for server/mini). A commit waits for a slot before CommitCompactVolume,
so a queued commit holds no volume lock and reads keep flowing. The
wait counts against the caller's deadline and returns the context
error if it expires; a slot granted to an already-expired context is
handed back, so a request the caller gave up on never starts a commit,
and maintenance mode is checked again once the slot is held.
The slot is released whether the commit succeeds or fails. Compaction
itself is not gated, so -maxParallelVacuumPerServer still sets reclaim
throughput. 0, the default, keeps today's behaviour.

The limit sits in the volume server so it covers every commit caller:
the master's vacuum sweep, the vacuum worker task and volume.tier.compact.

The Rust volume server mirrors the flag and behaviour: queued commits
wait on a semaphore keyed to the grpc-timeout deadline, hold no store
lock, recheck maintenance mode once a permit is held, and release it
on every exit path.

weed/command/server.go

@@ -149,6 +149,7 @@ func init() {
 	serverOptions.v.readMode = cmdServer.Flag.String("volume.readMode", "proxy", "[local|proxy|redirect] how to deal with non-local volume: 'not found|read in remote node|redirect volume location'.")
 	serverOptions.v.compactionMBPerSecond = cmdServer.Flag.Int("volume.compactionMBps", 0, "limit compaction speed in mega bytes per second")
 	serverOptions.v.maintenanceMBPerSecond = cmdServer.Flag.Int("volume.maintenanceMBps", 0, "limit maintenance (replication / balance) IO rate in MB/s. Unset is 0, no limitation.")
+	serverOptions.v.maxParallelVacuumCommits = cmdServer.Flag.Int("volume.compactionMaxParallelCommits", 0, "limit how many vacuum commits run at once on this server; queued commits hold no volume lock. 0 means unlimited")
 	serverOptions.v.fileSizeLimitMB = cmdServer.Flag.Int("volume.fileSizeLimitMB", 256, "limit file size to avoid out of memory")
 	serverOptions.v.ldbTimeout = cmdServer.Flag.Int64("volume.index.leveldbTimeout", 0, "alive time for leveldb (default to 0). If leveldb of volume is not accessed in ldbTimeout hours, it will be off loaded to reduce opened files and memory consumption.")
 	serverOptions.v.concurrentUploadLimitMB = cmdServer.Flag.Int("volume.concurrentUploadLimitMB", 0, "limit total concurrent upload size, 0 means unlimited")