Provide way to pass Descending Read option to the Walk command

While we were working on CLI story, we want to have retry behavior for when we fetch the "MostRecentEnvelope". To do this, we wanted to read envelopes in descending order using the walk function (which currently has retry logic).

We found descending read option not be supported under the current walk function. Current implementation sets the read options explicitly.

As a workaround, we will decorate the client.Read function which is passed to Walk(). We will have it add the descending read option, and get the one envelope we need.

As an alternative approach, we suggest you consider moving retry from Walk to the lower-level Read function.

CC: @bwasmith

Start with vendor/code.cloudfoundry.org/log-cache/pkg/client/walk.go and inspect Walk's explicit read options and the lower-level Read entry point. Compare the two approaches described in the issue, then confirm which API should support descending reads while retaining the needed retry behavior. Done means callers can read envelopes in descending order through the Walk-related API.