make unverified consensus ISOTime accessible through Tor's ControlPort

Currently only verified, accepted Tor consensus ISOTime is available.

Quote Tor control protocol:

     "consensus/valid-after"
     "consensus/fresh-until"
     "consensus/valid-until"
      Each of these produces an ISOTime describing part of the lifetime of
      the current (valid, accepted) consensus that Tor has.
      [New in Tor 0.2.6.3-alpha]

Unverified consensus ISOTime is unavailable.

This information is interesting in context for anonymity distributions and secure network time synchronization, usability and whatnot. Used by Tails' tordate or Whonix's anondate.

However, these tools rely on parsing Tor's log, which is fragile.

It would be nice, if something like

• consensus-unverified/valid-after
• consensus-unverified/fresh-until,
• and consensus-unverified/valid-until

where accessible through Tor's ControlPort.

      Each of these produces an ISOTime describing part of the lifetime of
      the unverified (invalid, rejected) consensus that Tor has.
      [New in Tor 0.2.7.x-...]

This feature requests completes the related one make certificate lifetime accessible through Tor's ControlPort (#16822 (moved)).

Use cases:

• clock slightly off: verified consensus (already implemented: #10395 (moved))
• clock more off: unverified consensus (this ticket)
• clock a lot off: certificate lifetime (#16822 (moved))