Rename SessionType to MediaKeySessionType.

This avoids potential naming collisions with other specifications.

Also, use simple <a> markup to reference enum types.
Bug 27111 - Separate "persistent" session type into "persistent-license" and "persistent-release-message".
Check for any existing unclosed session with a sessionId matching the one passed to load().

Also includes other minor changes related to session type.
Use hyphens in the MediaKeyMessageType value names.

This is consistent with the other enums and https://www.w3.org/Bugs/Public/show_bug.cgi?id=27317#c5.