This is an automated cherry-pick of #18321

What is changed and how it works?

Issue Number: Close #18317

1. Get memory limit multiple times is unnecessary.
2. The previous memory checks's granularity was too fine, performing a check on every chunk. Since Lightning writes in 16KB chunks, the frequent checks resulted in noticeable overhead.

What's Changed:

1. This PR change the behavior to only get memory limit once at beginning.
2. This PR adjusts the memory check to be performed per request instead of per chunk

sst_importer: Get Memory limit at the sst service initialization #18321

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

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

Release note

None