This PR ensures that we enter a lock inside the `@SendableProperty`
implementation as soon as we access a computed property. Additionally,
it mirrors the access level of the original property. Both changes are
required for [improved
accuracy](https://github.com/apple/container/pull/144) of progress
updates in container. Additionally, it should resolve
https://github.com/apple/containerization/issues/60 that occurs on
certain configurations.
Please tag as 0.1.1 after merging.