Fix batch reads hanging after digest mismatch retries are ignored

[void-ptr974]

Motivation

Batch reads could hang or fail after receiving a corrupt response from a bookie. The request was marked complete before digest verification, so although a digest mismatch triggered a retry
on another replica, the retry response could be ignored because the request was already completed.

Changes

• Verify all entries in a batch response before completing the read request.
• Keep the request incomplete on digest mismatch so the existing retry path can read the same batch from the next replica.
• Avoid retaining partially verified entries when one entry in the batch fails digest verification.
• Clarify the ByteBufList ownership and release paths with comments.

Tests

Added coverage for the batch-read digest mismatch retry path:

• testDigestMismatchRetriesNextReplicaAndCompletes

  • Replaces the first replica with a real bookie that corrupts read responses.
  • Verifies that a digest mismatch does not complete the batch request prematurely.
  • Confirms the same batch is retried from another replica and completes with the original entry data.

• testDigestMismatchAfterPartialVerificationDoesNotRetainEntries

  • Reads a two-entry batch where the first entry verifies successfully but the second entry is corrupted.
  • Verifies that no partially verified entries are retained after the digest mismatch.
  • Confirms the whole batch is retried from another replica and both returned entries match the original data.

Also ran the full TestBatchedRead suite to ensure existing batch-read behavior remains unchanged, including normal reads, partial batch responses, missing entries, and failed bookie
scenarios.

[void-ptr974]

@hangc0276 @horizonzy PTAL.