Don't deserialize AggregateAndProof to tree

[dapplion]

Describe the bug

Taking a CPU profile in Prater on a node with 1 validator 10.36% of CPU time is spent on tree_deserializeFromBytes.

I think we don't really need to deserialize to tree backed. We can manually cache the roots that we need.

The lifetime of an AggregateAndProof is relatively simple:

1. Gossip
2. Validation queue
3. Gossip validation
   3A. Verify AttestationData signature. Compute the AttestationData root and keep it.
   3B. Verify AggregateAndProof signature. Recycle AttestationData root to compute the signing root.
4. Op pool: Reuse the already computed AttestationData root
   4A. Block production
   4B. Pruned

A struct or even binary backed takes less memory, it's cheaper to deserialize and it's cheaper to access it's properties.