Add missing links between public values

[LindaGuiga]

This PR aims at implementing the missing public value links, especially for public values introduced by the receipts and logs PR:

• It applies reads for the transaction numbers, the cumulative gas used and block bloom filters (before and after the current transaction).
• It initializes block bloom filters (it creates a segment for the block bloom filters instead of using GlobalMetadata, to avoid code duplication, since each bloom filter is comprised of eight 32-bits words).
• It adds links in aggregation and block proofs for cum_gas_used, txn_number and block_filters.
• It also links state trie roots between block proofs and ensures that transaction and receipt tries are empty at the beginning of a block.

Please let me know if there's anything I missed, or any suggestions/corrections!

[wborgeaud]

Looks great!