DRTVWR-558: Introduce LLDispatchListener batched requests.

Now the value of the incoming event's dispatch key may be an LLSD::String (as
before), a map or an array, as documented in the augmented Doxygen class
comments. LLDispatchListener will attempt multiple calls before sending a
reply.

(cherry picked from commit 7671b37285c6cdf1afcddb0019311a822c8a4dc5)