We were doing intermediate step copies to Data objects both for push and
pull. We only need the data for the lifetime of the singular writes to
update the state of the checksums and to write to disk somewhere, so we
can use a view into the buffer from the http client to satisfy this.
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.
Danny Canter