Remove temporary boxed keys in batched_multi_get

[axnsan12]

By forcing the caller to give us an iterator of key references, we can avoid having to clone and allocate one Box per input key in batched_multi_get, since the keys are already guaranteed to live longer than the function call.

This is a slight breaking change, but IMO an acceptable one which should make any users of batched_multi_get happy - the only reason to be using batched_multi_get in the first place is to avoid allocations, otherwise one would already use the simpler multi_get, which allocates for both keys and return values.

Examples of breaking changes:

let keys: Vec<String> = vec!["k0".to_string(), "k1".to_string()];
// NOT ALLOWED: owned `Vec` of owned keys
let _ = db.batched_multi_get_cf(&cf, keys, true)
// ALLOWED: borrowed `Vec` of owned keys (`&Vec<T>` impls `IntoIterator<Item=&T>`)
let _ = db.batched_multi_get_cf(&cf, &keys, true)
// ALLOWED: iterator of references to keys
let _ = db.batched_multi_get_cf(&cf, keys.iter().take(1), true)

// NOT ALLOWED: iterator of owned keys
let _ = db.batched_multi_get_cf(&cf, (0..4).map(|i| i.to_string()), true)
// need to collect into temporary then pass references
let keys = (0..4).map(|i| i.to_string()).collect::<Vec<_>>();
let _ = db.batched_multi_get_cf(&cf, &keys, true)  // or `keys.iter()`

// STILL ALLOWED: owned `Vec` of `&[u8]` keys
let keys: Vec<&[u8]> = vec![b"k0", b"k1", b"k2"];
let _ = db.batched_multi_get_cf(&cf, keys, true)

[axnsan12]

Previous checks failed because clippy was right! Pushed a change to also remove the useless collect of keys into a temporary Vec<&[u8]>.