Starting block is never added to head_tracker

[michaelsproul]

Description

This is a slightly annoying inconcistency I encountered while working on Hydra: the genesis block isn't placed into the head_tracker when starting up from genesis.

The only place we insert into the head tracker is when importing a new block:

lighthouse/beacon_node/beacon_chain/src/beacon_chain.rs

Lines 3079 to 3080 in dfcb336

	self.head_tracker
	.register_block(block_root, parent_root, slot);

Because the genesis block isn't imported, and there's no special handling for it, it never makes it into the head_tracker.

There's one other register_block occurrence, but it's in the (broken) fork revert code which we don't expect to be called very often:

lighthouse/beacon_node/beacon_chain/src/builder.rs

Lines 668 to 669 in dfcb336

	// Update head tracker.
	head_tracker.register_block(block_root, block.parent_root(), block.slot());

I think the same logic applies for checkpoint sync. This may simplify the fix, as we might be able to just hack in a call to head_tracker.register_block for the head in BeaconChainBuilder::build.