Change the api of transaction apply_batch

[Erk-]

I propose to change the api of the apply_batch method in transactions from:

pub fn apply_batch(
    &self,
    batch: Batch,
) -> UnabortableTransactionResult<()>

to

pub fn apply_batch(
    &self,
    batch: &Batch,
) -> UnabortableTransactionResult<()>

This will then mean that it will be possible to write code like this:

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(())
})?;

[spacejam]

I like it! Thanks for improving this API :)