Content commit is updated to take in a context, allowing
content to be committed within the same context the writer
was in. This is useful when commit may be able to use more
context to complete the action rather than creating its own.
An example of this being useful is for the metadata implementation
of content, having a context allows tests to fully create
content in one database transaction by making use of the context.
Signed-off-by: Derek McGowan <derek@mcgstyle.net>
After some analysis, it was found that Content.Reader was generally
redudant to an io.ReaderAt. This change removes `Content.Reader` in
favor of a `Content.ReaderAt`. In general, `ReaderAt` can perform better
over interfaces with indeterminant latency because it avoids remote
state for reads. Where a reader is required, a helper is provided to
convert it into an `io.SectionReader`.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
