[trivial] Add missing const qualifiers.

Add some const qualifiers to references that are not modified and should
be marked as const.

src/main.cpp

@@ -1468,7 +1468,7 @@ bool GetTransaction(const uint256 &hash, CTransaction &txOut, const Consensus::P
     if (fAllowSlow) { // use coin database to locate block that contains transaction, and scan it
         int nHeight = -1;
         {
-            CCoinsViewCache &view = *pcoinsTip;
+            const CCoinsViewCache& view = *pcoinsTip;
             const CCoins* coins = view.AccessCoins(hash);
             if (coins)
                 nHeight = coins->nHeight;

src/txmempool.cpp

@@ -885,7 +885,7 @@ bool CTxMemPool::HasNoInputsOf(const CTransaction &tx) const
     return true;
 }
 
-CCoinsViewMemPool::CCoinsViewMemPool(CCoinsView *baseIn, CTxMemPool &mempoolIn) : CCoinsViewBacked(baseIn), mempool(mempoolIn) { }
+CCoinsViewMemPool::CCoinsViewMemPool(CCoinsView* baseIn, const CTxMemPool& mempoolIn) : CCoinsViewBacked(baseIn), mempool(mempoolIn) { }
 
 bool CCoinsViewMemPool::GetCoins(const uint256 &txid, CCoins &coins) const {
     // If an entry in the mempool exists, always return that one, as it's guaranteed to never

src/txmempool.h

@@ -672,10 +672,10 @@ private:
 class CCoinsViewMemPool : public CCoinsViewBacked
 {
 protected:
-    CTxMemPool &mempool;
+    const CTxMemPool& mempool;
 
 public:
-    CCoinsViewMemPool(CCoinsView *baseIn, CTxMemPool &mempoolIn);
+    CCoinsViewMemPool(CCoinsView* baseIn, const CTxMemPool& mempoolIn);
     bool GetCoins(const uint256 &txid, CCoins &coins) const;
     bool HaveCoins(const uint256 &txid) const;
 };