Support configurable s2n blinding delay about s2n-tls

[lrstewart]

It's possible that the 50 seconds came from the fact that shutdown used to potentially blind twice. But that hasn't been the case for a while: 7227160#diff-9ef39f857dd44706fab0fcbdafa024ca5c4a0cbe6184ffc11d68c1c7f3275f4e

[maddeleine]

For posterity, it looks like our delay is a random value from 10-30 seconds and the original justification was to have it match somewhat with a normal TCP keep-alive timer setting. That still may be too high for some applications, but it doesn't go as high as 50 seconds.