Don't crash authorities with more than 254 shared random reveals
Rather than asserting that a tor network never has more than 253 authorities, we could instead log when we truncate the number of reveals in the shared random hash.
As a consequential change, we should do this truncation as late as possible, so that the value assigned to
srv->num_reveals is not truncated.
In any case,
tor_assert(reveal_num < UINT8_MAX); is unnecessarily strict, it can be
tor_assert(reveal_num <= UINT8_MAX);.
Please see my branch sr-no-crash on https://github.com/teor2345/tor.git for fixes to these issues.