Fix two TCP RACK issues:

• Convert the TCP delayed ACK timer from ms to ticks as required. This fixes the timer on platforms with hz != 1000.
• Don't delay acknowledgements which report duplicate data using DSACKs.

Reviewed by: rrs@
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D21512