Specify ClearKey License Release format for persistent-license sessions

[mwatson2]

Clear Key supports temporary and persistent-usage-record sessions, but not persistent-license sessions.

Since this is an optional capability, it does not have interop implications if added to the specification now.

The only additional material, aside from stating that this session type is supported, is the format for the record of license destruction, which could simply be an object containing the list of key ids.

[mwatson2]

Do people feel this should be V1NonBlocking or VNext ?

[ddorwin]

The Clear Key section already says it is supported:

The "persistent-license" MediaKeySessionType: Implementations may support this type.

Although it is perhaps not explicitly stated, I think the License Release Format and License Release Acknowledgement Format sections may have originally been intended to be used for all session types that fire "license-release" messages. It was probably the addition of "firstTime" and "latestTime" to the License Release Format that makes this less obvious (and technically broken for "persistent-license").

I think this is a bug/regression in the V1 text and should be fixed. Since it doesn't really add anything, I think it's fine.

I suggest that we clarify that this is the format for all "license-release" messages and that the time members are only present for "persistent-release-message" sessions. We can then provide examples for each persistent session type.

[mwatson2]

PR #256 to specify the Clear Key license release format.