lib: Add per-message encryption with OpenSSL

This adds a per-message symmetric encryption using the OpenSSL
library. At flow allocation, an Elliptic Curve Diffie-Hellman exchange
is performed to derive a shared secret, which is then hashed using
SHA3-256 to be used as a key for symmetric AES-256 encryption. Each
message on an encrypted flow adds a small crypto header that includes
a random 128-bit Initialization Vector (IV). If the server does not
have OpenSSL enabled, the flow allocation will fail with an -ECRYPT
error.

Future optimizations are to piggyback the public keys on the flow
allocation message, and to enable per-flow encryption that maintains
the context of the encryption over multiple packets and doesn't
require sending IVs.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

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