Raise guard threshold to top 25% or 2000 kilounits

Authorities now assign the Guard flag to the fastest 25% of the
network (it used to be the fastest 50%). Also raise the consensus
weight that guarantees the Guard flag from 250 to 2000. For the
current network, this results in about 1100 guards, down from 2500.
This step paves the way for moving the number of entry guards
down to 1 (proposal 236) while still providing reasonable expected
performance for most users.

Implements ticket 12690.

changes/ticket12690

@@ -0,0 +1,9 @@
+  o Minor features:
+    - Authorities now assign the Guard flag to the fastest 25% of the
+      network (it used to be the fastest 50%). Also raise the consensus
+      weight that guarantees the Guard flag from 250 to 2000. For the
+      current network, this results in about 1100 guards, down from 2500.
+      This step paves the way for moving the number of entry guards
+      down to 1 (proposal 236) while still providing reasonable expected
+      performance for most users. Implements ticket 12690.
+

src/or/config.c

@@ -147,7 +147,7 @@ static config_var_t option_vars_[] = {
   V(AuthDirInvalid,              LINELIST, NULL),
   V(AuthDirInvalidCCs,           CSV,      ""),
   V(AuthDirFastGuarantee,        MEMUNIT,  "100 KB"),
-  V(AuthDirGuardBWGuarantee,     MEMUNIT,  "250 KB"),
+  V(AuthDirGuardBWGuarantee,     MEMUNIT,  "2 MB"),
   V(AuthDirReject,               LINELIST, NULL),
   V(AuthDirRejectCCs,            CSV,      ""),
   V(AuthDirRejectUnlisted,       BOOL,     "0"),

src/or/dirserv.c

@@ -1588,7 +1588,7 @@ dirserv_compute_performance_thresholds(routerlist_t *rl,
     /* (Now bandwidths is sorted.) */
     if (fast_bandwidth_kb < ROUTER_REQUIRED_MIN_BANDWIDTH/(2 * 1000))
       fast_bandwidth_kb = bandwidths_kb[n_active/4];
-    guard_bandwidth_including_exits_kb = bandwidths_kb[(n_active-1)/2];
+    guard_bandwidth_including_exits_kb = bandwidths_kb[n_active*3/4];
     guard_tk = find_nth_long(tks, n_active, n_active/8);
   }
 
@@ -1646,7 +1646,8 @@ dirserv_compute_performance_thresholds(routerlist_t *rl,
 
   if (n_active_nonexit) {
     guard_bandwidth_excluding_exits_kb =
-      median_uint32(bandwidths_excluding_exits_kb, n_active_nonexit);
+      find_nth_uint32(bandwidths_excluding_exits_kb,
+                      n_active_nonexit, n_active_nonexit*3/4);
   }
 
   log_info(LD_DIRSERV,