vine: remove transfer pairs with sources

[btovar]

Avoid segfault with race condition where source worker disconnects before it can be blamed from a failed transfer.

Post-change actions

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• make test Run local tests prior to pushing.
• make format Format source code to comply with lint policies. Note that some lint errors can only be resolved manually (e.g., Python)
• make lint Run lint on source code prior to pushing.
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Additional comments

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