Make one authority connection early so the client can check its clock.
Redo the analysis for the new timing schedule.
Add IPv4 and IPv6 alternation scheme for clients that have both an
IPv4 and IPv6 address.
Add retry timer maximum and retry timer reset events.
Include min and max fallback directory weights.
To implement #4483 we need to contact multiple directory mirrors
to increase bootstrap reliability. This patch implements the
exponential backoff suggested in
https://trac.torproject.org/projects/tor/ticket/4483#comment:22
The patch also analyses the reliability of the new scheme, and
compares it to the current Tor implementation.
