Enable caching of key-derived context info such as key schedules from

Enable caching of key-derived context info such as key schedules from
one encryption operation to another.  Use a new function in the
enc_provider structure for cleanup.  Implement caching of aes_ctx
values.

Using Greg's performance tests from the derived-key caching work, on a
2.8GHz Xeon, I see 1 million AES-128 encryptions of 16 bytes improved
by 5-6%; encryptions of 1024 bytes and checksums are not significantly
affected.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23574 dc483132-0cff-0310-8789-dd5450dbe970