br restore failed when injection tikv network delay

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
1、run br restore
2、injection tikv network delay 50ms

### 2. What did you expect to see? (Required)
br restore can succeed

### 3. What did you see instead (Required)
br restore failed
[br.log.2024-10-28T03.50.17Z.zip](https://github.com/user-attachments/files/17595733/br.log.2024-10-28T03.50.17Z.zip)

`[32mfatal error: too many concurrent timer firings

runtime stack:
runtime.throw({0x6d50fce?, 0xc000a841c0?})
/usr/local/go/src/runtime/panic.go:1067 +0x48 fp=0x7fa572ffcae8 sp=0x7fa572ffcab8 pc=0x22e3788
runtime.(*timer).unlockAndRun(0xc00198b8d0, 0x0?)
/usr/local/go/src/runtime/time.go:1076 +0x28e fp=0x7fa572ffcb50 sp=0x7fa572ffcae8 pc=0x22cb26e
runtime.(*timers).run(0xc00009b190, 0x8180b97e8b7382)
/usr/local/go/src/runtime/time.go:1008 +0xf0 fp=0x7fa572ffcb78 sp=0x7fa572ffcb50 pc=0x22caf90
runtime.(*timers).check(0xc00009b190, 0x100000004?)
/usr/local/go/src/runtime/time.go:942 +0x13d fp=0x7fa572ffcbc0 sp=0x7fa572ffcb78 pc=0x22cadbd
runtime.findRunnable()
/usr/local/go/src/runtime/proc.go:3270 +0x6f fp=0x7fa572ffcd38 sp=0x7fa572ffcbc0 pc=0x22b164f
runtime.schedule()
/usr/local/go/src/runtime/proc.go:3995 +0xb1 fp=0x7fa572ffcd70 sp=0x7fa572ffcd38 pc=0x22b3411
runtime.park_m(0xc0020856c0)
/usr/local/go/src/runtime/proc.go:4102 +0x1eb fp=0x7fa572ffcdc8 sp=0x7fa572ffcd70 pc=0x22b382b
runtime.mcall()
/usr/local/go/src/runtime/asm_amd64.s:459 +0x4e fp=0x7fa572ffcde0 sp=0x7fa572ffcdc8 pc=0x22ea7ee
goroutine 1 gp=0xc0000061c0 m=nil [semacquire, 15 minutes]:
runtime.gopark(0x2274b5e?, 0x7fa59067a8e8?, 0x60?, 0xc6?, 0x22de100?)
/usr/local/go/src/runtime/proc.go:424 +0xce fp=0xc007c60488 sp=0xc007c60468 pc=0x22e38ae
runtime.goparkunlock(...)
/usr/local/go/src/runtime/proc.go:430
runtime.semacquire1(0xc01048de50, 0x0, 0x1, 0x0, 0x12)
/usr/local/go/src/runtime/sema.go:178 +0x225 fp=0xc007c604f0 sp=0xc007c60488 pc=0x22bf5a5
sync.runtime_Semacquire(0x67aab80?)
/usr/local/go/src/runtime/sema.go:71 +0x25 fp=0xc007c60528 sp=0xc007c604f0 pc=0x22e5265
sync.(*WaitGroup).Wait(0xc006c4d920?)
/usr/local/go/src/sync/waitgroup.go:118 +0x48 fp=0xc007c60550 sp=0xc007c60528 pc=0x22ffc68
golang.org/x/sync/errgroup.(*Group).Wait(0xc01048de40)
/root/go/pkg/mod/golang.org/x/sync@v0.8.0/errgroup/errgroup.go:56 +0x25 fp=0xc007c60570 sp=0xc007c60550 pc=0x3458165
github.com/pingcap/tidb/br/pkg/restore/snap_client.(*SnapClient).restoreSSTFilesInternal(0xc00216aea0, {0x75faa10, 0xc0021ee0f0}, {0xc006db0000, 0xb62, 0xc010456a80?}, {0x75fa888, 0xc006d12340})
/workspace/source/tidb/br/pkg/restore/snap_client/tikv_sender.go:451 +0x2d8 fp=0xc007c60680 sp=0xc007c60570 pc=0x58bf378
github.com/pingcap/tidb/br/pkg/restore/snap_client.(*SnapClient).RestoreSSTFiles(0xc00216aea0, {0x75faa10, 0xc0021ee0f0}, {0xc006db0000, 0xb62, 0x1ed9}, {0x75fa888, 0xc006d12340})
/workspace/source/tidb/br/pkg/restore/snap_client/tikv_sender.go:395 +0xa8 fp=0xc007c606e0 sp=0xc007c60680 pc=0x58bf028
github.com/pingcap/tidb/br/pkg/restore/snap_client.(*SnapClient).RestoreTables(0xc00216aea0, {0x75faa10, 0xc0021ee0f0}, {0x75ce718, 0xb184560}, {0xc005fad200, 0x20, 0x20}, {0xc0046fc000, 0x1ed9, ...}, ...)
/workspace/source/tidb/br/pkg/restore/snap_client/tikv_sender.go:316 +0x5f5 fp=0xc007c60870 sp=0xc007c606e0 pc=0x58be6d5
github.com/pingcap/tidb/br/pkg/task.runSnapshotRestore({0x75faa10, 0xc001485b80}, 0xc001bc7600, {0x761b580, 0xc0018c6620}, {0x6ce70f5, 0x10}, 0xc001cea588, 0x0)
/workspace/source/tidb/br/pkg/task/restore.go:1095 +0x2d85 fp=0xc007c610b0 sp=0xc007c60870 pc=0x59c5425
github.com/pingcap/tidb/br/pkg/task.RunRestore({0x75faa10, 0xc001485b80}, {0x761b580, 0xc0018c6620}, {0x6ce70f5, 0x10}, 0xc001cea588)
/workspace/source/tidb/br/pkg/task/restore.go:704 +0x465 fp=0xc007c619b8 sp=0xc007c610b0 pc=0x59c1a05
main.runRestoreCommand(0xc0019cdb08, {0x6ce70f5, 0x10})
/workspace/source/tidb/br/cmd/br/restore.go:75 +0x74a fp=0xc007c61b90 sp=0xc007c619b8 pc=0x5d9a50a
main.newDBRestoreCommand.func1(0xc0019cdb08?, {0xc0019c4800?, 0x4?, 0x6cbb44b?})
/workspace/source/tidb/br/cmd/br/restore.go:195 +0x1f fp=0xc007c61bb8 sp=0xc007c61b90 pc=0x5d9cb3f
github.com/spf13/cobra.(*Command).execute(0xc0019cdb08, {0xc0000740e0, 0x8, 0x8})
/root/go/pkg/mod/github.com/spf13/cobra@v1.8.1/command.go:985 +0xaaa fp=0xc007c61d40 sp=0xc007c61bb8 pc=0x587c94a
github.com/spf13/cobra.(*Command).ExecuteC(0xc001534608)
/root/go/pkg/mod/github.com/spf13/cobra@v1.8.1/command.go:1117 +0x3ff fp=0xc007c61e18 sp=0xc007c61d40 pc=0x587d27f
github.com/spf13/cobra.(*Command).Execute(...)
/root/go/pkg/mod/github.com/spf13/cobra@v1.8.1/command.go:1041
main.main()`

### 4. What is your TiDB version? (Required)
./br -V
Release Version: v8.4.0-alpha-475-g7292117502
Git Commit Hash: 729211750239904fbfdd78eba99cc3e8f754d5c6
Git Branch: HEAD
Go Version: go1.23.2
UTC Build Time: 2024-10-27 06:11:32
Race Enabled: false