Find root cause of zero RTT calculations

We added a band-aid in https://github.com/hashicorp/consul/issues/3789, but still need to figure out the root cause.

It turns out from reports from another user this isn't Windows-specific, so in some set of circumstances memberlist is calculating a zero RTT for a probe. Did a thorough review of the code and it seems like the monotonic time info should be preserved, so it's not clear what's going on.

Start with issue #3789 and the memberlist probe RTT calculation path described here. Investigate the circumstances that produce a zero RTT and trace whether monotonic time data is preserved through that path. Done means the root cause is identified and documented, with a focused regression test or reproduction if the investigation supports one.

Written by the indexing model from the issue text.