blockstore: Allocate rocksdb keys more efficiently

[steviez]

Problem

The Blockstore stores different data types in different columns. More so, the key type might vary column to column. For example, the key type for a SlotMeta is just a Slot as shown here:

agave/ledger/src/blockstore.rs

Lines 497 to 500 in 7629a15

	/// Returns the SlotMeta of the specified slot.
	pub fn meta(&self, slot: Slot) -> Result<Option<SlotMeta>> {
	self.meta_cf.get(slot)
	}

Under the hood, rocksdb just wants a &[u8] for the key. So, we have a function for turning the type (Slot from our example) into a Vec<u8>:

agave/ledger/src/blockstore_db.rs

Line 772 in 7629a15

fn key(index: Self::Index) -> Vec<u8>;

Thus, we'll perform an extra allocation to transform the key into a byte array for every get/put operation.

Summary of Changes

• Refactor all key conversion to occur in LedgerColumn
• Add a KEY_LEN field to Column trait
• Replace instances of C::key() (which allocates a Vec) to the following form (which will get allocated on the stack since K is known at compile time):

        let mut key_buffer = [0u8; K];
        C::serialize_key(&mut key_buffer, key);

• Add MultiGetBytes helper to only perform one Vec allocation for multi_get() calls, instead of one Vec per key + Vec to hold all keys

Performance

This item isn't the worst offender in the flamegraphs. But after adding in a temporary counter, I see that tip-of-master is calling C::key() ~18.5k times per second on mnb. Each of those calls is allocating a Vec, so shaving all those heap allocations is a win