Add a SPD cache to speed up lookups.

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