Fix deferred credits

[Eitu33]

• document all added functions
• try in sandbox / simulation / labnet
• unit tests on the added/changed features
  • make tests compile
  • make tests pass
• add logs allowing easy debugging in case the changes caused problems

[AurelienFT]

@Eitu33 Can you add some description on the changes and why so that we can review ?

[Eitu33]

@AurelienFT @sydhds What this PR aims to fix:

1. Deferred credits changes were build to be absolute changes, meaning that the apply of speculative changes to the final deferred credits must override the values for any given existing entry. This is not what was done, the previous behaviour was accumulating the credits what would result in credits slashes never being applying because the change value was set to 0 and that 0 was later summed to the final value instead of replacing it.
2. The execution of deferred credits only credited the coins equivalent to the given address without setting the credit values to 0, what is even worst is that the only pruning operation we perform on credits removes only the credits that reached 0. The ram was increasing indefinitely because of that.
3. The previous deferred credits bootstrap didn't take into consideration that credits preceding the execution slot could change, with the denunciations coming they can, so the credits had to parallelly stream its changes in case some happened during the main credits bootstrap process.