Change stats serialization

[bchamagne]

Description

Serialize nb_transaction & burned_fees on 64 bits instead of 32 bits.
Fixes #1059

Type of change

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

How Has This Been Tested?

• Tested with data from mainnet
• Tested with hotreload after a benchmark

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

[Neylix]

I think this fix could be added in 1.2.2 milestone, since this bug could make transaction mint reward refused with consensus error if a node restart between self repair and mint reward time

[samuelmanzanera]

@bchamagne can you change then the migration file to match 1.2.2 ?