Persistency or lock for animations

[jakebruce]

Issue: when publishing Commands and Animations from the same node, sometimes a Command will override an Animation due to a race condition and the Animation never gets played.

Proposed solution: add an optional "lock duration" field to the Animation message, and that Animation will have precedence over all Command requests for the specified duration. Another option might be to provide this as a service instead, to maintain the synchronous/asynchronous distinction between services and topics.