getRandomHexChars(): use /dev/urandom just to seed.
getRandomHexChars(): use /dev/urandom just to seed.

On Darwin /dev/urandom depletes terribly fast. This is not an issue
normally, but with Redis Cluster we generate a lot of unique IDs, for
example during nodes handshakes. Our IDs need just to be unique without
other strong crypto requirements, so this commit turns the function into
something that gets a 20 bytes seed from /dev/urandom, and produces the
rest of the output just using SHA1 in counter mode.

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis