Fork manager handlers precedence

[igorcrevar]

Description

There was a problem with the way the fork manager stores handlers when multiple forks start on the same block.
We should maintain the precedence of handlers, and their precedence is determined by the order of their registration.

For example:

forkManager.RegisterHandler(ForkA, HandlerA, func() string { return "A" })
forkManager.RegisterHandler(ForkB, HandlerA, func() string { return "B" })
forkManager.RegisterHandler(ForkC, HandlerA, func() string { return "C" })

If all of these forks (ForkA, ForkB, ForkC) start on the same block and all of them are activated, the GetHandler function should return the last one that was registered. If the last one is not activated or is deactivated, then the second one should be returned.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually