Some further cleanup in WriteBatchWithIndex::MultiGetFromBatchAndDB (#12143)

Some further cleanup in WriteBatchWithIndex::MultiGetFromBatchAndDB (#12143)

Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12143

https://github.com/facebook/rocksdb/pull/11982 changed `WriteBatchWithIndex::MultiGetFromBatchDB` to preallocate space in the `autovector`s `key_contexts` and `merges` in order to prevent any reallocations, both as an optimization and in order to prevent pointers into the container from being invalidated during subsequent insertions. On second thought, this preallocation can actually be a pessimization in cases when only a small subset of keys require querying the underlying database. To prevent any memory regressions, the PR reverts this preallocation. In addition, it makes some small code hygiene improvements like incorporating the `PinnableWideColumns` object into `MergeTuple`.

Reviewed By: jaykorean

Differential Revision: D52136513

fbshipit-source-id: 21aa835084433feab27b501d9d1fc5434acea609