Use C++11 thread-safe static initializers in coins.h/coins.cpp

Taken from Bitcoin bitcoin#8173/888483098e60f2a944f1d246bbfec4d14a2975f8

Needed to align code for upcoming per-utxo backports.

Author: codablock

src/coins.cpp

@@ -56,11 +56,7 @@ void CCoinsViewBacked::SetBackend(CCoinsView &viewIn) { base = &viewIn; }
 bool CCoinsViewBacked::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) { return base->BatchWrite(mapCoins, hashBlock); }
 CCoinsViewCursor *CCoinsViewBacked::Cursor() const { return base->Cursor(); }
 
-SaltedTxidHasher::SaltedTxidHasher()
-{
-    GetRandBytes((unsigned char*)&k0, sizeof(k0));
-    GetRandBytes((unsigned char*)&k1, sizeof(k1));
-}
+SaltedTxidHasher::SaltedTxidHasher() : k0(GetRand(std::numeric_limits<uint64_t>::max())), k1(GetRand(std::numeric_limits<uint64_t>::max())) {}
 
 CCoinsViewCache::CCoinsViewCache(CCoinsView *baseIn) : CCoinsViewBacked(baseIn), hasModifier(false), cachedCoinsUsage(0) { }
 

src/coins.h

@@ -269,7 +269,7 @@ class SaltedTxidHasher
 {
 private:
     /** Salt */
-    uint64_t k0, k1;
+    const uint64_t k0, k1;
 
 public:
     SaltedTxidHasher();