Fix issue in payload disposing and add unit test for asking multiple times by the same payloadId

[jmederosalvarado]

Changes:

• Fix an issue that caused the BlockImprovementContext.CancellationTokenSource to be disposed multiple times
• Add unit test to verify we can ask multiple times using the same payloadId without errors

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• 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

Requires testing

• Yes
• No

In case you checked yes, did you write tests??

• Yes
• No

[LukaszRozmej]

This is actually a downgrade. I think we want to stop improvements on first get.

[LukaszRozmej]

Pushed fixes, we still want to stop improving block after first get, but Dispose should be implemented in a way that allows multiple calls. This is .Net requirement, from docs:

If an object's Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times.