index hashtable: run random indices through siphash

If /dev/urandom is a NOBUS RNG backdoor, like the infamous Dual_EC_DRBG,
then sending 4 bytes of raw RNG output over the wire directly might not
be such a great idea. This mitigates that vulnerability by, at some
point before the indices are generated, creating a random secret. Then,
for each session index, we simply run SipHash24 on an incrementing
counter.

This is probably overkill because /dev/urandom is probably not a
backdoored RNG, and itself already uses several rounds of SHA-1 for
mixing. If the kernel RNG is backdoored, there may very well be
bigger problems at play. Four bytes is also not so many bytes.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

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