[Bug] Alter table failed when alter task cannot find the deleted tablet on be because of balance

[caiconghui]

Describe the bug
Some alter operation on big table is easily failed, because balance operation on tablet is running all time, and may cause some tablet deleted, which would cause alter task cannot find deleted tablet

To Reproduce

1. create a big table with many tablet
2. alter schema for that table at some interval to reproduce the problem (it happened on our environment)
   Expected behavior
   Alter operation on table succeed
   Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.