Allow alter index queries to be queued/deferred

Currently, alter index queries are returned when the indexer has been made aware of the task to complete. This means that scripting a lot of index moves using the cbq file options is not possible, since it may try to start moving another index while one is already in the process of being moved.

We should be able to queue up these tasks, similar to how defer_build currently works, and execute them in the background when it is possible to do so. This would allow for mass change scripts to run successfully instead of it being a very manual process of executing each command one at a time and waiting for it to be completed before running the next.