Fix potential race condition leading to nil dereference

[sthelen-enqs]

Split ApproveOrDeny*Limit public functions into a public part which takes a msgCounter and resolves it to a Message using the pendingLimits as before and a private part which takes a *Message and can be called directly from the internal WriteApprovalCallbacks. This removes the need to call FeatureLocalInterface.ApproveOrDenyWrite with an "empty" message when a message is not found in the pendingLimits for ApprovalRequests that don't match the current use case.

Fixes #104