Enrich write batch

[v01dstar]

This is a cherry-pick of https://github.com/tikv/rocksdb/pull/177

Adding WriteBatch::Iterator and WriteBatchInternal:: AppendContents().

WriteBatchInternal::AppendContents() is used by TiKV to efficiently merge multiple write batches into a larger batch, thereby reducing overhead. This operation is particularly useful for avoiding unnecessary memory copies or decoding of data.

WriteBatch::Iterator is not being directly used by TiKV. But it is still kept.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Connor1996, glorv, LykxSassinator

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[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-09-26 08:01:18.42983109 +0000 UTC m=+1725748.170255028: :ballot_box_with_check: agreed by Connor1996.
• 2024-09-27 03:10:53.828916709 +0000 UTC m=+1794723.569340648: :ballot_box_with_check: agreed by LykxSassinator.
• 2024-09-27 06:32:38.463294094 +0000 UTC m=+1806828.203718053: :heavy_multiplication_x::repeat: reset by v01dstar.
• 2024-09-27 07:07:52.860935664 +0000 UTC m=+1808942.601359615: :ballot_box_with_check: agreed by LykxSassinator.
• 2024-09-27 07:11:21.471349686 +0000 UTC m=+1809151.211773624: :ballot_box_with_check: agreed by glorv.