Capture ExecutionContext after every binding invoke

[obligaron]

Fixes #120

Reqnroll (and Specflow v4,x) uses Async internally (only one code path).
To ensure the user sees no difference between the old (synchronous) implementation and the new ones the ExecutionContext (including AsyncLocal) are handled manually by Reqnroll.
This is done using ExecutionContext.Capture() in BindingDelegateInvoker,InvokeInExecutionContext.

The problem is that this only works once.
Background:

• In the first call
  • ExecutionContextHolder.Value is null.
  • This means that ExecutionContext.Run is not called.
  • The Binding-Method is called with the current ExecutionContext and modifies the AsyncLocal.
  • Current ExecutionContext is captured
  • => everything is fine.
• In the second call
  • ExecutionContextHolder.Value is not null.
  • ExecutionContext.Run is called.
    • ExecutionContext.Run captures the current ExecutionContext
    • In ExecutionContext.Run the Binding-Method is called and modifies the AsyncLocal.
    • The ExecutionContext.Run restores the captured "old current" ExecutionContext with the old values
  • The current (restored) ExecutionContext is captured
  • => Information is lost.

Bufix: Capture ExecutionContext explicitly in ExecutionContext.Run after every binding invoke.

Types of changes

• Bug fix (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).
• Performance improvement
• Refactoring (so no functional change)
• Other (docs, build config, etc)

Checklist:

• I've added tests for my code. (most of the time mandatory)
• I have added an entry to the changelog. (mandatory)
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[gasparnagy]

The idea and the unit tests are great, I made a suggestion for simplifying the code.

[gasparnagy]

Thx!

[gasparnagy]

@obligaron you should be now on the @reqnroll/core-team (you need to accept invitation), so pushing new branches to the main repo should work now. (I hope. Please report if not.)