Support pinned builds with SCM Polling.

Supports the Populate option 'Pin build at Perforce Label' (or change),
and only triggers a build if the label is changed.
Minor improvements.

- Protection against undefined workspaces
- Logging improvements for Perforce
- Performance for Perforce connections
Expand labels for pinned builds.
Null protection for pin at label.
Log build version in p4java connection.