consider greasing the Retry mechanism

[marten-seemann]

With #4248, we can now set a limit on how many unvalidated handshakes we run before performing a Retry. With #4253, the application loses the API to trigger sending of Retries.

I'm concerned that this will lead to Retry to not getting exercised at all. We could randomly trigger a Retry on x% of QUIC handshakes, independent of any limits. This probably doesn't need to be configurable, but we could have another environment variable to turn it off (or to change the fraction). For example,QUIC_GO_RETRY_GREASING=0.05 would send Retries on 5% of connections.