Change Hashing from H3 to Siphash.

This commit mostly changes the hash function that is used for Internal
hashing of data < 36 bytes from H3 to Siphash. This change is motivated
by the fact that it turns out that H3 apparently does not deliver a very
good source of data uniqueness; running HLL with H3 as a hashing
function results in quite poor results (up to of 75% off in my tests).
In difference, running HLL with Siphash (or HMAC-MD5) changes this
factor to ~2%.

This also fixes a long-standing bug in Hash.h which truncated our hash
values to 32 bit on most machines.

Furthermore, it once again fixes a problem with the Rank function in
HLL.

src/Hash.h

@@ -81,7 +81,8 @@ protected:
 
 	void* key;
 	int is_our_dynamic;
-	int size, hash;
+	int size;
+	hash_t hash;
 };
 
 extern void init_hash_function();