[scudo] lower quarantine default sizes

[cryptoad]

Summary

After discussing the current defaults with a couple of parties, the consensus
is that they are too high. 1Mb of quarantine has about a 4Mb impact on PSS, so
memory usage goes up quickly.
This is obviously configurable, but the default value should be more
"approachable", so both the global size and the thread local size are 1/4 of
what they used to be.

[alekseyshl]

LGTM

lib/scudo/scudo_flags.cpp

void initFlags() {
   // Override from environment.
   ScudoParser.ParseString(GetEnv("SCUDO_OPTIONS"));
 
   InitializeCommonFlags();
 
   // Sanity checks and default settings for the Quarantine parameters.
 
   if (f->QuarantineSizeMb < 0) {
-    const int DefaultQuarantineSizeMb = FIRST_32_SECOND_64(16, 64);
+    const int DefaultQuarantineSizeMb = FIRST_32_SECOND_64(4, 16);
     f->QuarantineSizeMb = DefaultQuarantineSizeMb;
   }
   // We enforce an upper limit for the quarantine size of 4Gb.
   if (f->QuarantineSizeMb > (4 * 1024)) {
     dieWithMessage("ERROR: the quarantine size is too large\n");
   }
   if (f->ThreadLocalQuarantineSizeKb < 0) {
     const int DefaultThreadLocalQuarantineSizeKb =
-        FIRST_32_SECOND_64(256, 1024);
+        FIRST_32_SECOND_64(64, 256);
     f->ThreadLocalQuarantineSizeKb = DefaultThreadLocalQuarantineSizeKb;
   }
   // And an upper limit of 128Mb for the thread quarantine cache.
   if (f->ThreadLocalQuarantineSizeKb > (128 * 1024)) {
     dieWithMessage("ERROR: the per thread quarantine cache size is too "
                    "large\n");
   }
   if (f->ThreadLocalQuarantineSizeKb == 0 && f->QuarantineSizeMb > 0) {