Use constant time memcmp() when comparing packet HMACs.

This eliminates a timing side channel vulnerability, which could
potentially allow an attacker to compute a valid HMAC, and insert arbitrary
ciphertext data into the connection. If an attacker also identifies packets
with a plaintext it can guess (e.g. small packets every 1s are probably
pings), the attacker can xor the ciphertext to mangle the packet to
arbitrary plaintext. Although this type of attack is rarely seen in the
wild, it is generally considered technically viable.

Signed-off-by: Steffan Karger <steffan@karger.me>