Change transaction apply_batch to take argument by-reference.

[Erk-]

This change facilitates that you can setup a batch and then execute it
inside of a transaction. For example:

let db = config.open().unwrap();
let t1 = db.open_tree(b"1")?;

let mut b1 = Batch::default();
b1.insert(b"k1", b"v1");
b1.insert(b"k2", b"v2");

t1.transaction(|tree| {
    tree.apply_batch(&b1)?;
    Ok(())
})?;

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This PR is the implementation of #1013 if it is accepted.

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I did have a spurious fail in the test log_writebatch, but I am unsure if
it is connected to this change in any way.

[spacejam]

@Erk- this looks good, thanks for improving this :) I'll add an entry to the CHANGELOG mentioning this breaking change for the upcoming 0.32 release.