blockstore: Defer multi_get_bytes() allocation to caller

[steviez]

Problem

multi_get_bytes() currently returns owned memory (Vec<u8>). But, the callers of multi_get_bytes() only require borrowed memory (&[u8]).

Summary of Changes

So, update multi_get_bytes() to return a reference to rocksdb owned memory; the caller can copy to owned memory if they require it

Future Work

Some similar work can likely be done with other methods that call .get_bytes(). For example, this one here:

agave/ledger/src/blockstore.rs

Lines 2266 to 2268 in e48672c

	pub fn get_data_shred(&self, slot: Slot, index: u64) -> Result<Option<Vec<u8>>> {
	let shred = self.data_shred_cf.get_bytes((slot, index))?;
	let shred = shred.map(ShredData::resize_stored_shred).transpose();

However, the call to ShredData::resize_stored_shred requires owned memory, but is only applicable to legacy data shreds as far as I know. Additional rework beyond just changing the underlying rocksdb type will be necessary, so I plan to tackle those in a follow-on PR