When large SPDs are used, we face two problems:

- too many CPU cycles are spent during the linear searches in the SPD
  for each packet
- too much contention on multi socket systems, since we use a single
  shared lock.

Main changes:

- added the sysctl tree 'net.key.spdcache' to control the SPD cache
  (disabled by default).
- cache the sp indexes that are used to perform SP lookups.
- use a range of dedicated mutexes to protect the cache lines.

Submitted by: Emeric Poupon <emeric.poupon@stormshield.eu>
Reviewed by: ae
Sponsored by:	Stormshield
Differential Revision: https://reviews.freebsd.org/D15050