Use atomic operations in database to harden against failures

[michaelsproul]

Description

At present, each insert into the store is its own operation, but we often rely on invariants to be maintained between objects inserted at different times (e.g. if we have a block, we expect to have its state too). Through the use of transactions/atomics at the level of the KV store, we should be able to better maintain these invariants under adverse conditions (the process getting SIGKILLed, power outage, etc).

[adaszko]

Candidates for using atomic operations:

lighthouse/beacon_node/store/src/state_batch.rs

Lines 39 to 46 in cd0b86f

	pub fn commit<S: Store<E>>(self, store: &S) -> Result<(), Error> {
	self.items.into_iter().try_for_each(|item| match item {
	BatchItem::Full(state_root, state) => store.put_state(&state_root, state),
	BatchItem::Summary(state_root, summary) => {
	store.put_state_summary(&state_root, summary)
	}
	})
	}

lighthouse/beacon_node/beacon_chain/src/beacon_chain.rs

Lines 1475 to 1476 in cd0b86f

	self.store.put_state(&state_root, state)?;
	self.store.put_block(&block_root, signed_block)?;

lighthouse/beacon_node/beacon_chain/src/beacon_chain.rs

Lines 1823 to 1826 in cd0b86f

	for (block_hash, state_hash, slot) in abandoned_blocks.drain() {
	self.store.delete_block(&block_hash.into())?;
	self.store.delete_state(&state_hash.into(), slot)?;
	}