Implementation of guard node proposal:

* Switch to one guard per client
** Look into decide_num_guards().
** See if setting NumEntryGuards=1 breaks any assumption of the rest of the code.
** Implementation depends on whether we pick alternative behavior of section 1.1.1 or not.

* Increase guard rotation period
** Play with MIN_GUARD_LIFETIME etc.
** Switch GuardLifetime consensus option in authorities
** Implementation depends on whether we pick alternative behavior of section 1.2.1 or not.

* Raise bw thresholds for guard
** See set_routerstatus_from_routerinfo() and AuthDirGuardBWGuarantee etc.
** How to test:
   Make fake network and see which guards are eligible to become guards


* Prioritize young guards for non-guard tasks
** Download and parse old consensuses in an external script,
   write file with results,
   have little-t-tor read the results.
   See how bw-auths do this.

** Bandwidth authorities work like this:
   dirserv_read_measured_bandwidths()
   https://gitweb.torproject.org/torflow.git/blob/HEAD:/NetworkScanners/BwAuthority/README.spec.txt#l333

** How to test:
*** How to test the external script:
    Make artificial data sets of consensuses and see if the external script parses them well
**** Test multiple data sets. (Guard in all the consensuses / Guard in no consensus / Guard in a few consesuses)
*** How to test Tor:
    Create artificial guard age results file, give the test a fake network (some relays) and see if Tor generates the right bandwidth weights.