Fix the shared secret keystore loading

[ghost]

Description

Because of the new DB design, the order of transaction is not desc but
asc.

So the loader had to be changed, to load the latest transaction
Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

When you run the node after few times, when you restart it should have error of proof_of_election, due to an invalid last shared secrets tx.
Also unit tests have been update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules