tcp: Congestion control move to using reference counting.

In the transport call on 12/3 Gleb asked to move the CC modules towards
using reference counting to prevent folks from unloading a module in use.
It was also agreed that Michael would do a user space utility like tcp_drop
that could be used to move all connections that are using a specific CC
to some other CC.

This is the half I committed to doing, making it so that we maintain a refcount
on a cc module every time a pcb refers to it and decrementing that every
time a pcb no longer uses a cc module. This also helps us simplify the
whole unloading process by getting rid of tcp_ccunload() which munged
through all the tcb's. Instead we mark a module as being removed and
prevent further references to it. We also make sure that if a module is
marked as being removed it cannot be made as the default and also
the opposite of that, if its a default it fails and does not mark it as being
removed.

Reviewed by: Michael Tuexen, Gleb Smirnoff
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D33249