Add WriteBatch SliceParts APIs with user-defined timestamp

[sherriiiliu]

Current WriteBatch SliceParts APIs does not accept user-defined timestamp, and the following description of current APIs is not true.

  // The following Put(..., const Slice& key, ...) API can also be used when
  // user-defined timestamp is enabled as long as `key` points to a contiguous
  // buffer with timestamp appended after user key. The caller is responsible
  // for setting up the memory buffer pointed to by `key`.

This patch adds new APIs for SliceParts that accept user-defined timestamp as this PR