hyperv/timesync: Refine the timesync policy.

[t-yachen_microsoft.com]

Summary

• Differentiate sync request from sample request.
  
  Sync request will sync the time immediately without checking the drift. Sample request will sync the time only when drift exceeds some threshold.
• Add tunable sysctls for both requests.
  
  hw.hvtimesync.sample_drift: if set to 0, ignore sample requests, otherwise, sync the time only when drift exceeds some threshold.
  
  hw.hvtimesync.ignore_sync_req: if set to 1, ignore sync requests.
• Remove unnecessary task queue and data structures.
  
  Implement the policy in hv_adj_guesttime directly.