Add support for polling without needing a workspace

This feature/improvement also avoids two problems:
1. If some jobs use TFS as an SCM, are tied to certain nodes and none of those are available, the plugin would decide that because no workspace was available (for polling), it should queue a build to create one.
2. In certain cases, asking the workspace what changeset it was at was returning erroneous results, causing the polling to think there still hadn't been a build for a recently-committed changeset.