move some Avatar management to AvatarHashMap
add a convenience method to AvatarHashMap to check for avatar removal
kill avatars in hash map after a timeout threshold
fix avatar kill behaviour
add a method to determine if avatar with display name exists
add an AvatarHashMap to the Agent
optionally add an AvatarHashMap instance to the ScriptEngine
add debug for erase from AvatarHashMap, repair check for display name
rename AvatarHash to AvatarList
Merge pull request #2691 from birarda/agent-avatar-manager

expose AvatarHashMap to Agent for Avatar querying from assignment-client