Add close call to the Snapshotter interface

A close method on the Snapshotter interface would be pretty useful for custom snapshotters that need to cleanup any resources. Currently there isn't an easy way to determine when the snapshot is finished when using a custom snapshotter. I've tried a couple workarounds, polling jmx and performing cleanup in the Snapshotter#shouldStream() method since it is called once after the snapshot phase. I dislike polling JMX because it is not always available, as is the case when using GraalVM. While these workarounds technically work for most setups, the addition of a Snapshotter#close() method would be much easier to work with.