fix iter may not be Released in some cases

[zier-one]

What problem does this PR solve?

When error happened in loop, iter may not be released. And when l0Num >= l0Trigger, we better release the iter, it may be affect compaction operation.

What is changed and how it works?

Check List

Tests

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

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update the tidb-ansible repository
• Need to be included in the release note

[zier-one]

/run-all-tests

[july2993]

LGTM
A not release iter will not affect compaction, but the files it references will not be deleted even it's compacted. Because the iter creates a snapshot to get a consistent view of data, once it's released and the reference count of the file drop to be 0 the file will be delete.

Besides goleveldb use runtime.SetFinalizer to make sure the Release() will be called...so even in the error case it will be Release finally.

[july2993]

LGTM

[zier-one]

@suzaku PTAL

[suzaku]

Rest LGTM.

[zier-one]

/run-all-tests

[zier-one]

/run-all-tests

[suzaku]

LGTM