Don't use flawed way of generating random keys.

libdevcore/FixedHash.cpp

@@ -25,4 +25,4 @@
 using namespace std;
 using namespace dev;
 
-std::mt19937_64 dev::s_fixedHashEngine(time(0));
+std::random_device dev::s_fixedHashEngine;

libdevcore/FixedHash.h

@@ -31,7 +31,7 @@
 namespace dev
 {
 
-extern std::mt19937_64 s_fixedHashEngine;
+extern std::random_device s_fixedHashEngine;
 
 /// Fixed-size raw-byte array container type, with an API optimised for storing hashes.
 /// Transparently converts to/from the corresponding arithmetic type; this will