Fix SIGTERM not behaving the same as SIGINT

[asdacap]

Resolves #4057

Changes:

• Hold process exit handler until app finish cancelling tasks.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe):

Testing

Run "kill -s SIGTERM pidof Nethermind.Runner", confirmed final log now show up like SIGINT.

[LukaszRozmej]

Isn't it a problem that we have 2xWait for 1xSet?

[asdacap]

We do? I thought its basically a Countdownlatch. https://docs.microsoft.com/en-us/dotnet/api/system.threading.manualresetevent?view=net-6.0 so it's suppose to work for that.

[LukaszRozmej]

sorry got confused