Require fallbacks to have 90% Running, V2Dir, and Guard flags

This allows 73% of clients to bootstrap in the first 5 seconds without
contacting an authority.

Part of #18828.

changes/fallbacks-201612

@@ -5,12 +5,10 @@
       updateFallbackDirs.py.
       Closes ticket 20878.
     - Require fallback directories to have the same address and port for
-      4 months. (4 months is a tradeoff between the expected time between
-      major Tor releases, which is 6 months, and the number of relays with
-      enough stability.) Relays whose OnionOO stability timer is reset on
-      restart by bug 18050 should upgrade to Tor 0.2.8.7 or later, which has
-      a fix for this issue.
-      Closes ticket 20880; removes short-term fix in e220214 in
+      7 dayss. (Due to the number of relays with enough stability.) Relays
+      whose OnionOO stability timer is reset on restart by bug 18050 should
+      upgrade to Tor 0.2.8.7 or later, which has a fix for this issue.
+      Closes ticket 20880; maintains short-term fix in e220214 in
       tor-0.2.8.2-alpha.
     - Make it easier to change the output sort order of fallbacks.
       Closes ticket 20822.

scripts/maint/updateFallbackDirs.py

@@ -147,9 +147,9 @@ MAX_LIST_FILE_SIZE = 1024 * 1024
 ADDRESS_AND_PORT_STABLE_DAYS = 7
 # What time-weighted-fraction of these flags must FallbackDirs
 # Equal or Exceed?
-CUTOFF_RUNNING = .95
-CUTOFF_V2DIR = .95
-CUTOFF_GUARD = .95
+CUTOFF_RUNNING = .90
+CUTOFF_V2DIR = .90
+CUTOFF_GUARD = .90
 # What time-weighted-fraction of these flags must FallbackDirs
 # Equal or Fall Under?
 # .00 means no bad exits