Refactor state JsonRPC endpoint

[lazovicff]

Since all the LT and Seed entries are sorted inside BTreeMap, we should simply loop over its entries until we reach a specified PAGE_SIZE.

Steps for 2 level LT BTreeMap:

• Open an iterator over BTreeMap outer entries (from keys)
• Open an iterator over inner entries (OutboundLocalTrust.outbound_trust_scores keys)
• Take entries until we reach PAGE_SIZE.
• Create a token indicating where we left of, a touple of (from, to)
• Repeat

For Seed BTreeMap, it is much simpler since it is 1 level:

• Iterate over entries and take PAGE_SIZE
• Create a token indicating where we left of, a singular value