hyperv/hn: Set the TCP ACK/data segment aggregation limit

Set TCP ACK append limit to 1, i.e. aggregate 2 ACKs at most. Aggregating
anything more than 2 hurts TCP sending performance in hyperv. This
significantly improves the TCP sending performance when the number of
concurrent connetion is low (2~8). And it greatly stabilizes the TCP
sending performance in other cases.

Set TCP data segments aggregation length limit to 37500. Without this
limitation, hn(4) could aggregate ~45 TCP data segments for each
connection (even at 64 or more connections) before dispatching them to
socket code; large aggregation slows down ACK sending and eventually
hurts/destabilizes TCP reception performance. This setting stabilizes
and improves TCP reception performance for >4 concurrent connections
significantly.

Make them sysctls so they could be adjusted.

Reviewed by: adrian, gallatin (previous version), hselasky (previous version)
Approved by: adrian (mentor)
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5185