raise AuthDirFastGuarantee from 20KB to 100KB

This patch reverts part of 39ceda7e05d7 (where it used to be 100KB).

changes/bug4489

@@ -0,0 +1,8 @@
+  o Major bugfixes (performance):
+    - Remove the artificially low cutoff of 20KB to guarantee the Fast
+      flag. In the past few years the average relay speed has picked
+      up, and while the "top 7/8 of the network get the Fast flag" and
+      "all relays with 20KB or more of capacity get the Fast flag" rules
+      used to have the same result, now the top 7/8 of the network has
+      a capacity more like 32KB. Bugfix on 0.2.1.14-rc. Fixes bug 4489.
+

src/or/config.c

@@ -184,7 +184,7 @@ static config_var_t _option_vars[] = {
   V(AuthDirBadDir,               LINELIST, NULL),
   V(AuthDirBadExit,              LINELIST, NULL),
   V(AuthDirInvalid,              LINELIST, NULL),
-  V(AuthDirFastGuarantee,        MEMUNIT,  "20 KB"),
+  V(AuthDirFastGuarantee,        MEMUNIT,  "100 KB"),
   V(AuthDirGuardBWGuarantee,     MEMUNIT,  "250 KB"),
   V(AuthDirReject,               LINELIST, NULL),
   V(AuthDirRejectUnlisted,       BOOL,     "0"),