block signature rollup

[dabasov]

To implement rollups we need a way to store block's validity fact. To prove the validity of the block we use signature aggregated during voting phase is is exactly what we are going to use as block's validity fact. We can go further and prune all transaction signatures as they become obsolete when majority of nodes voted for this block and validated it.
To implement this change we need:

1. Add signature aggregate to block
2. Prune signatures
3. Update block's signature every time we add new block, since added block can contain QC, we can simply put this signature to local QC.block and prune it's transactions
4. When validating block, first we check it's signature aggregate and if absent validate transaction signatures as usual, if signature is correct we prune transaction's signatures.
5. When synchronizing blockchain, we must do the same operation