Remove `blk` field from BlockRef

[arnetheduck]

nimbus-eth1/execution_chain/core/chain/forked_chain/chain_branch.nim

Line 20 in fe162f0

blk* : Block

stores a full copy of each block in memory - this block is used very rarely, ie only during replays.

A second copy of the same data is stored in txFrame as the block is serialized and persisted there after verification.

When there is a longer period of non-finality, this leads to high memory consumption. Instead:

• remove blk from BlockRef and load it on demand from txFrame - this halves memory usage
• eventually, flatten txFrame and keep it up to date using the heads structure, akin to how FcState does it - this way, almost no block payload data will be kept in memory.

#3628 implements this idea the other way around by keeping blk and removing it from txFrame - that approach however has almost no advantages and cannot be flattened.