In the process of trying to bring down the time of sending an OMEMO
encrypted message I noticed that a lot of time was spent in generating the
random key and IV (~0.54s on my machine).
By changing this to a single call to `gcry_random_bytes_secure()` I have
been able to slash the time by 50% to ~0.27s.
Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>
d9a8c579b6