tcp_hostcache: use SMR for lookups, mutex(9) for updates.

In certain cases, e.g. a SYN-flood from a limited set of hosts,
the TCP hostcache becomes the main contention point. To solve
that, this change introduces lockless lookups on the hostcache.

The cache remains a hash, however buckets are now CK_SLIST. For
updates a bucket mutex is obtained, for read an SMR section is
entered.

Reviewed by: markj, rscheff
Differential revision: https://reviews.freebsd.org/D29729