Store only block proof for shards in persistent state

[Rexagon]

For now we are using the same code for masterchain and other shards to prepare shard state on cold boot. It requires a full block, its proof and queue diff. For mc block we already have a proof which we received from a getKeyBlocks stuff and we are downloading a block data to have a full config and shard descriptions. But for other shards we only need to check a state_update.new against a downloaded state root.

I made a temp fix (#412) to preserve all persistent blocks for now, but it may slow down the blocks GC.