Improve default strhash function

Summary:
from kevinweil@gmail.com in opensource

Test Plan:
compile

DiffCamp Revision: 109728
Reviewed By: jsong
CC: agiardullo, jsong, groys, scribe-dev@lists
Revert Plan:
OK

git-svn-id: svn+ssh://tubbs/svnapps/fbomb/trunk/fbcode/scribe@26719 2248de34-8caa-4a3c-bc55-5e52d9d7b73a

src/env_default.h

@@ -73,7 +73,16 @@ class integerhash {
 class strhash {
  public:
   static uint32_t hash32(const char *s) {
-    return 0;
+    // Use the djb2 hash (http://www.cse.yorku.ca/~oz/hash.html)
+    if (s == NULL) {
+      return 0;
+    }
+    uint32_t hash = 5381;
+    int c;
+    while ((c = *s++)) {
+      hash = ((hash << 5) + hash) + c; // hash * 33 + c
+    }
+    return hash;
   }
 };