fix: use db when it has been opened

[acelyc111]

What problem does this PR solve?

• It's a bug to call rocksdb::CancelAllBackgroundWork when close replica after db open failed
• Explicitly delete db in pegasus_server_impl's destructor

What is changed and how it works?

No related.

Check List

Tests

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

Code changes

• Has exported function/method change
  No
• Has exported variable/fields change
  No
• Has interface methods change
  No
• Has persistent data change
  No

Side effects

• Possible performance regression
  No
• Increased code complexity
  No
• Breaking backward compatibility
  No

Related changes

• Need to cherry-pick to the release branch
  Yes
• Need to update the documentation
  No
• Need to be included in the release note
  No