Fix potential data race in pendingItem.done()

[gunli]

Fixes

Master Issue: #1332

Motivation

Return the buffer to the pool after all callbacks have been called

Modifications

Return the buffer to the pool after all callbacks have been called

Verifying this change

• Make sure that the change passes the CI checks.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable / docs / GoDocs / not documented)
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• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[gunli]

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