Make relay integrity digests harder to guess by padding cells with random bytes
The tor spec says we should put random bytes in padding cells: https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1534
But we don't currently do this (see #22948 (moved)). And we don't put random bytes in other cells.
This makes it easier to guess the circuit integrity digest, which makes some DoS and malleability attacks easier.
Should we pad all cells with random bytes?