This shaves about 7% off our per-cell AES crypto time for me; the
effect for accelerated AES crypto should be even more, since the AES
calculation itself will make an even smaller portion of the
counter-mode performance.
(We don't want to do this for pre-1.0.0 OpenSSL, since our AES_CTR
implementation was actually faster than OpenSSL's there, by about
10%.)
Fixes issue #4526.
This thing was pretty pointless on versions of OpenSSL 0.9.8 and later,
and almost totally pointless on OpenSSL 1.0.0.
Also, favor EVP by default, since it lets us get hardware acceleration
where present. (See issue 4442)