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
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D7707