Prevent TCP Cubic to abruptly increase cwnd after slow-start

Introducing flags to track the initial Wmax dragging and exit
from slow-start in TCP Cubic. This prevents sudden jumps in the
caluclated cwnd by cubic, especially when the flow is application
limited during slow start (cwnd can not grow as fast as expected).
The downside is that cubic may remain slightly longer in the
concave region before starting the convex region beyond Wmax again.

Reviewed by: chengc_netapp.com, tuexen (mentor)
Approved by: tuexen (mentor), rgrimes (mentor, blanket)
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D23655