Issues with insertion into `mainExplorationQueue`

[ifndefJOSH]

How best to insert? We need to make sure enqueueSuccessors is called in stateToIdCallback, so we will have to bind our callback method within the exploration thread. Additionally, the method must exist within that exploration thread, so some code moving around will be necessary.

[ifndefJOSH]

As of commit 4a04a07, we create a separate stateToIdCallback for each thread. Does not yet compile though.

[ifndefJOSH]

Note that the stateToIdCallback we create links to the exploredStates() method.