only move the queue items if the new item index is less than size

[SMovaghati]

• [ ] Does this PR have a descriptive title that could go in our release notes?
• [ ] Does this PR add any new dependencies?
• [ ] Does this PR modify any existing APIs?
• [ ] Is the change to the API backwards compatible?
• [ ] Should this result in any changes to our documentation, either updating existing docs or adding new ones?

What does this implement/fix? Briefly explain your changes.

Before, it used to always move the queue items, but that is not necessary if the item to add is at the end of the queue (and queue still has capacity).

[mmthomas]

The code as is currenly correct; there is no rush to merge this change. However, it does allocate extra memory in the vector to support the current insertion strategy, and it keeps track of its own size and capacity.

We have a larger improvment to this whole class that simplifies the logic, reduces the memory requried, and removes the need for and explicit memmove, allowing std:vector to manage the insertion optimally.

@SMovaghati we should bring in all of our changes to this class instead of this small change.