This is an automated cherry-pick of #9905

What problem does this PR solve?

Issue Number: close #9906

Problem Summary:

If profiles.default.init_thread_count_scale to 0, tiflash stuck at loading metadata because we try to add task into a thread pool with num_thread == 0.

What is changed and how it works?

* If `profiles.default.init_thread_count_scale == 0`, run `loadMetadata` and `BgStorageInit` in serial order.
* Respect `terminate_signals_counter` in `BGStorageInit`

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  deploy a cluster and add this config line to tiflash by tiup edit-config

    profiles.default.init_thread_count_scale: 0

[2025/02/24 16:14:18.240 +08:00] [INFO] [BgStorageInit.cpp:181] ["Lazily init store"] [thread_id=1]
[2025/02/24 16:14:18.361 +08:00] [INFO] [BgStorageInit.cpp:138] ["Init stores without thread pool"] [thread_id=77]
[2025/02/24 16:17:35.796 +08:00] [INFO] [BgStorageInit.cpp:158] ["Storage inited finish. total_count=60258 init_count=57697 error_count=0 terminated=0 datatype_fullname_count=24"] [thread_id=77]

• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None