Handle concurrent access to a pipeline's status field

[hariso]

Description

Closes #1778.

The approach that we initially planned didn't quite work and was changed to use a sync.Mutex because of following:

1. Having an exported field (and this is part of the "public" Conduit API) that's an atomic is sub-optimal user experience because it forces the user to work with the low-level constructs. In other words we should either expose a field or methods that work with just the pipeline.Status field.
2. With a field, it's possible to simply overwrite it: myPipeline.Status = atomic.Pointer{}
3. We still need to provide a getter and setter because otherwise working with atomic.Pointer would involve a few steps:

   // set status
   myPipeline.Status.Store(lang.Ptr(theStatus))
   // get status
   theStatus := *myPipeline.Status.Load()

4. A pointer, we need to make sure the field is properly initialized.

For the reasons above, this PR uses a sync.Mutex and adds a getter and setter method for the status field, that is now unexported.

Quick checks

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.

[raulb]

@hariso LGTM! I only posted a non blocking question.