Add nrt_utils commands to get/set/list backend resource versions

[aprudhomme]

Adds new cli commands to nrt_utils for interacting with the version backend resource (global state, index state, nrt point state, warming queries)

getResourceVersion

Get the current version for a resource. This is the version stored in the _current object in S3 that references the current resource value object. If there is no current resource, not_set is returned.

setResourceVersion

Set the current version for a resource. This sets the value stored in the _current object in S3 that references the current resource value object. The command checks that the new version object exists before updating the reference.

listResourceVersions

List the resource values from S3. A versionPrefix can be provided to filter by the time string prefix. Only the first 1000 entries are printed. I think this is ok for now and the prefix can be used to drill down to certain time ranges.