Use SipHash-2-4 for CCoinsCache index

This is ~1.7x slower than the Lookup3-of-Xor-with-salt construct we were
using before, but it is a primitive designed for exactly this.

src/coins.h

@@ -8,6 +8,7 @@
 
 #include "compressor.h"
 #include "core_memusage.h"
+#include "hash.h"
 #include "memusage.h"
 #include "serialize.h"
 #include "uint256.h"
@@ -264,21 +265,22 @@ public:
     }
 };
 
-class CCoinsKeyHasher
+class SaltedTxidHasher
 {
 private:
-    uint256 salt;
+    /** Salt */
+    uint64_t k0, k1;
 
 public:
-    CCoinsKeyHasher();
+    SaltedTxidHasher();
 
     /**
      * This *must* return size_t. With Boost 1.46 on 32-bit systems the
      * unordered_map will behave unpredictably if the custom hasher returns a
      * uint64_t, resulting in failures when syncing the chain (#4634).
      */
-    size_t operator()(const uint256& key) const {
-        return key.GetHash(salt);
+    size_t operator()(const uint256& txid) const {
+        return SipHashUint256(k0, k1, txid);
     }
 };
 
@@ -295,7 +297,7 @@ struct CCoinsCacheEntry
     CCoinsCacheEntry() : coins(), flags(0) {}
 };
 
-typedef boost::unordered_map<uint256, CCoinsCacheEntry, CCoinsKeyHasher> CCoinsMap;
+typedef boost::unordered_map<uint256, CCoinsCacheEntry, SaltedTxidHasher> CCoinsMap;
 
 /** Cursor for iterating over CoinsView state */
 class CCoinsViewCursor