hyperv/timesync: Rework time adjustment policy

- By default, adjust time upon SYNC request.  It can be disabled
  through hw.hvtimesync.ignore_sync_req.  SYNC request will be
  sent by hypervisor the host is resumed, rebooted, etc.
- By default, adjust time upon SAMPLE request, if there is 100ms
  difference between VM time and hypervisor time.  This can be
  disabled through hw.hvtimesync.sample_drift.

And nuke the unnecessary task, since channel callback is running
in a Hyper-V taskqueue nowadays.

Submitted by:   YanZhe Chen <t-yachen microsoft com>
Discussed with: Dexuan Cui <decui microsoft com>, Hongjiang Zhang <honzhan microsoft com>, sephe
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D7707