Closing partitionVertexIdPool when VertexIDAssigner is closed

> One of the two threadpools opened in VertexIDAssigner is never shutdown. This came up in my case when running JMH benchmarks. At the end of the benchmarks the threads would still be around after a close was issued.

A [PR](https://github.com/JanusGraph/janusgraph/pull/667) was submitted to fix this a while back, but an issue was not created to track the problem reported. The PR was merged and then subsequently [reverted](https://github.com/JanusGraph/janusgraph/pull/810) because it was breaking the build.

Start by locating VertexIDAssigner and its close path, then inspect how partitionVertexIdPool is opened and shut down. Review PR 667 and its reversion in PR 810 for prior context, and use the JMH benchmark scenario mentioned in the issue to check for lingering threads. Done means closing VertexIDAssigner shuts down both thread pools without breaking the build.

Written by the indexing model from the issue text.