Minor improvements to proactive scanning

[jbearer]

• Add a major_scan_offset parameter to make it easy to configure a group of nodes so that they all do their major scans at different times
• Log major scans at WARN level so we see it in deployments
• Add a tracing instrument to scans with relevant information

Motivated by this pattern:

The spikes are exactly every hour on all query nodes and correspond with major scans.